Going back to post #7...
Instrument Housing Grounds:
These 2 Grounds (one each side) must be connected to the back of each, Tachometer and Speedometer housings.
These 2 grounds are part of the Main Dash harness originating from the LH pillar also shown in post #7.
If you still have your ground jumper available from the negative battery post pull one of the instrument lamp Gray wires out.
With bulb still in the socket pull headlight **** out, touch your ground jumper to the metal part of the bulb socket, the light should shine.
If not it is a voltage "or bulb" problem not a ground problem.

 

Ok. I’ll give it a shot. I’ve had the grounds plugged in on the tach and Speedo.

 

My car had some water leak issues and caused some corrosion / rust that was obvious at the fuse panel.
I know that this may sound like it wont make any difference, but....... After I tried just about everything, and replaced and checked all the fuses. I pulled them out again, one at a time. I used a tiny wire brush with DeoxIT D5 spray at the fuse panel where the fuses insert, used a tiny bit of dielectric grease on the fuses and put them back. I thought that it wouldn't make any difference, and I hate to admit that it, but it cured 3 of my 4 issues. I would do this before I pull the fuse box. Disconnect the battery when you do this.

My issues were power to the light switch and no gauge cluster or center gauge lights. I know that you have a brake light issue, but some of these circuits are effected by others.

 

Thank you. I’ll go to the Napa tomorrow and buy that stuff.

 

I did what you said and still no light. I’m gonna try what Prestige593 said next. ⬇️

Tomorrow I’m going to go to Napa auto parts and get that stuff. If that doesn’t work Im gonna probably end up taking the fuse panel off the fire wall to see how the connections look.

 

To use the meter on the socket while it is plugged in, connect your black to gnd, connect a slightly unbent paper clip to the red. You can then go through the same place in the back of the socket the wires go and test while hot. If you are hot at green, then its fuse block. If it isnt hot, then you need to check the power coming in to the sw that feeds that rheostat, which would be brown.
When you check the fuse block, meter at the tabs that hold the fuse, not the fuse. If the fuse panel got wet, it is possible that there is enough corrsion on the tabs to stop electricity, OR worse case, the connectors that attach the wires to the hot side of the fuse block in the rear are rotted off.
If you have power at green on the sw, and nothing at the fuse block, then take power off, switch your meter to resistance (ohms) and meter between the hot side of the block and the green wire at the connector. Anything much over zero "0" you have a problem.
I am working on the same thing, but my rheostat wire was broken on the switch. Let you know if mine works after the new sw goes in.

 

Varmintmist,

Thanks. I’m out of town for a few days. When I get back I’m gonna give that a shot. I look forward to hearing if you get yours working. I’ll be sure to update soon.

 

let there be light

custom rear of the socket checker. If its to deep or small for a test light or meter lead, adjust. I chased mine back to the switch from the fuse looking for voltage. Had good in at the sw, nothing out.

The culprit


cant even see the open unless you move it.
I also learned that the new sw sits about 1/64th deeper than my **** wanted to go. pulled the pin out of the **** (musclefucked it) but after that the pin seated in the switch. Put the **** back on with a dab of RTV in it for now. I will probably put in a set screw later if it falls off while I am using it. Not on the list of need right now things.

 

Varmintmist,

So, if I’m understanding what you said, you tested power coming into the switch, and you had power, but going out of the switch,(green wire?) you didn’t have power. Your **** wouldn’t fit in your switch all the way, and didn’t fit perfectly with the pin that holds the **** in, so you jacked around with the pin, and now the switch activates the light?

 

Yes, the part that applies to your problem at the moment is the testing.
You need to find out if you have power at the green wire at the switch because the power for the interior lights leaves the sw there. I had no power leaving the switch on the green wire but I had power coming in. I pulled the switch and found the rheostat broken.

IF you have 12vdc there, (green wire, lights on, rheostat turned up) you have to move downstream to the next place which is fuse panel.
IF you do not have 12vdc at the fuse panel on the tabs that hold the fuse in, then your problem is between those tabs and the switch. Pull the fuse and clean with sandpaper the tabs in the fuse holder, read it again. If you cant get the voltage, then pull the fuse block and have a look behind it.

Now
IF you do NOT have voltage on the green wire at the switch (green wire, lights on rheostat turned up) then you have to go upstream to the brown wire and check for voltage.
If you dont have voltage there.. follow the guide Prestige593 posted and head back upstream to orange then back to the fuse panel.

Does that make sense? Working backwards it is downstream end = bulb/ground, grey to fuse, through fuse, green from fuse to sw, brown feeding green through sw, and orange feeding brown through sw, then orange through fuse at the upstream end.

Question, do you have tail lights and not brake lights or just no break lights? Because the guide posted, shows that the brown also feeds the tail lights as well as powers interiors.
As to dielectric grease, yes, in all things at all times. It wont solve problems, but it will keep them from happening again.

The **** not fitting is just an annoying little anecdote.

 

Yes. That makes sense. I have brake lights when the headlight switch is off. As soon as I pull the switch to just turn on the dash lights brake lights and tail lights turn off. Same thing when it’s all the way pulled. Ever since I got it running when the headlights are on the brake lights and tail lights don’t turn on. It’s just in the last few weeks the dash lights crapped out. I’ve checked all grounds and they all work. So it’s gotta be on the hot side. The new fuse burned out on the new tail fuse. I replaced it. I put a tester on one connection and the other needle on the fuse and I had power pass though. The connections are a slight bit rusted on the right side of the fuse clip. I got emery paper and 320 grit sand paper to clean it up when I get back to town. I didn’t think that would be the problem because power flowed through it with the tester. I suppose it could be my problem. Ive tried everything these people suggested except the dielectric grease and deox spray, I’m gonna do that. And of course what you said. So I’ll try it when I get back to town. I really do appreciate everyone’s help and yours. Thanks.

 

If needed...
Remember to disconnect battery, #12 BUS is HOT anytime the battery is connected!
Here is a photo of what to expect when/If you need to split the 2 half's of the fuse panel, this is a Late 77 but looking at the Willcox 76 line diagram it is the same configuration.
The DK.GRN wire from headlight switch lands on the Right fuse clip (#6) power through the fuse then out (#5 Gray wire) and the BUS to the LPS socket...

 

I’m my eyes I’ve made massive progress with this. Yesterday I pulled just the tail light fuse and sanded it down. The result is I weird. The back right tail light works when the headlights are on, but still no brake lights. Here’s the other weird part, when I turn the headlights off only the back left brake light turns on. So it’s probably a contact problem on the fuse board.

The dash lights still don’t work, I chased all the head light wires and they’re all good, no obvious breaks at all. I sanded down the socket points because they where a little corroded. Today I’m gonna pull every fuse and sand down the points. I think we’ve narrowed it down to that, and to messing around with the headlight switch.

 

Randomly the dash lights started working, i have no idea why or anything but here's a list of what i did:
New fuses
dielectric grease
new ground strap under drivers side door jam
new light bulbs
pulled the fuse Pannel and cleaned all contacts
new headlight switch
new headlight switch wiring
chased all wires

That's all I can remember off the top of my head, I'm sure there's more but that's all I remember.
Here's a link to how i got the brake lights and running lights working again:
76 Vette Brake Lights Don’t Work When Headlamps Are On - CorvetteForum - Chevrolet Corvette Forum Discussion

Thank you so much for all of your suggestions. It helps sooooo much.

 

Guessing it was the moisture problem.

Dampness caused many minor problems that added up. Sometimes you have to just keep at it until magically you succeed.