Well I'm helping my buddy with his c3, its been a weekend event pretty much every weekend. Today we are trying to get the rear brake lights and turn signals working, The side markers and driving lights on the rear work but we can't get the rear turn signals or brake lights to function. We replaced the turn signal switch in the column hoping that would solve the problem being as it had chaffed wires but unfortunately it did not. We figured well maybe its a chassis ground, tried to hunt down the chassis ground under the left rear cross member but I can't find it, he cant find it. Pulling our hair out on this one. Front lights are fine. We disconnected the brown and black wires going to this rubber connector that appears to lead to rear light wiring harness and it did indeed kill the driving lights with that pulled, but that wire disappears behind a frame member directly behind the driver side outer most taillight, we filled the connector with dielectric grease and put it back together so he can at least flip his driving lights on and off to let people know he's braking on the way home. (apparently for some odd reason that brown and black wire in the rubber connector go up to the fuel sending unit, when thats disconnected the running lights dont work how the @#@%@ doesn't make any much sense to me)

At first before we got into changing the turn signal switch we simply changed the fused which was blown and the lights worked, turn signal, hazards, brake lights, then as soon as we went down the road they died but the fuse didn't go... any help would be appreciated. I should add that all flasher relays have been replaced

 

You need a chassis wiring diagram for the car, and a test light. I would first verify ground to the taillight brake circuit... you can do this with a simple alligator clip jumper from any socket to something close like a fuel tank strap bolt... if it lights up you search out the ground circuit. Otherwise simply follow the brake "juice" from the battery through each connection downstream until you lose power... voila!!!! there's your open.

 

If you have running lites your ground is good.
Pull the left and right rear light assemblies and remoce the light sockets and bulbs. You can easily verify for juice that way. If no juice inside the socket, probe the wires just ahead.
sometimes these old sockets just go bad, replacements at most parts houses.

Also clean the harmonica connector at the steering column these often have poor contacts. There is another connector at the drivers door jamb.

 

Those are definitely some great ideas. I guess at the least we can spray some DX5 in the harmonica connector, We will definitely put it in the game plan to check each individual light socket for juice, I have about three different wiring diagrams and none of them have been entirely too useful.

Ima need to get a few things straight for the next round of c3 wiring, From the harmonica connector on the column I am imagining their is probably a run of wire that would disappear into this connector that I was not aware existed at the driver side door jam which would then in theory go to that connector with the brown and black wire up and behind the rear driver tire to the rear tail light panel harness? I'm pretty sure that if the rear parking/driving lights are working then the ground for that section of harness is somewhat properly terminated. Bad sockets definitely need ruled out before anything too radical happens. I just had a pretty good idea, I might have to fish that run of wires out of the back end of the vette and test it on a bench.. Then we wont be so frustrated

 

the rear harness goes under the driver's sill plate and over the wheel hump. Just check for damage.
Don't pull the complete rear harness out. That's a real PITA.
Jack up the rear as high as you can and put it on jack stands.
Sit on a creeper and starting at the pass side free up all the wiring so it will drop down by the alarm horn. You will also have extra wires for the gas gauge and alarm horn.
I've found them just as easy to do by stripping all the tape, resoldering and heat shrinking all the connections, replacing bad sockets and retaping.
Will last a lifetime. Takes a couple hours.

Here is the redone lite harness of a 77.

 

Now that looks alot friendly to work on just hanging like that. I'll keep you guys updated on what we find!