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What year is your car? on late year c3's there are circite boards on the back of the speedo/tack and on the back of the center cluster. It would be money ahead to replace them.
I'll add to this just a tiny bit.. While you are in there.. find the brightest light you have and put it in the engine compartment. Shut the hood and from inside the car look for light... Any hole you find is a invite for heat to get in the car. I'm not saying you will find any, but it is a quick way to check. You might be surprised.
I would also check the condition of the Lower pillars behind the kick panels, and this is a great time to also check the condition of the fuse holders in the fuse box. Look for rusted holders and you can separate the box to remove them for cleaning if needed.
The heater core installation is a 3.5 hour job with the dash in the car and about a 1 hour job with it out.. so this really is a good idea to consider.
Replace the dash speakers with good quality 4"x6" speakers [they will still work well for mid-range/high-frequency sound. Check out the heater/A/C system for proper function on door servos, seals, hoses, etc. Check out all the gauge functions and repair if necessary. Replace ALL light bulbs in the dash area. Lubricate and/or replace the speedo and tach cables. Reskin the dash pad if the vinyl is damaged at all; re-dye it, if it's still in good shape.
This might seem kinda goofy but, with the dash out your brake booster mounting studs would be easier to get at. If you're thinking of painting or replacing the booster, now would be a good time.
this is a great time to also check the condition of the fuse holders in the fuse box. Look for rusted holders and you can separate the box to remove them for cleaning if needed.
Willcox
I'm replacing the wiring harness, that's why the dash is off
My '76 had a 1 inch gap in the vertical duct work going to the defrost. I fixed it by fabricating some plastic and bolting it to the duct to seal it. Hopefully yours will be ok but I kinda doubt it. The area is right behind where the Radio sits.
All great suggestions, but it truly astounds me how fast these projects get out of control. This one started with the removal of a dash and now includes a harness, bulbs, heater core, insulation and the worst part is that it all makes sense.