Confused
 

Have a 80 Corvette, L82 350ci

Replaced burnt out right rear brake light. When emergency flasher used, all lights flash on/off.

Without side markers or headlight's on, left / right turn signal lights all flash. When I turn on courtesy or headlights , both turn signals flash except right rear brake light which just stays on ?

When I apply the brakes, rear lights work .

I replaced flasher, but doing the same thing.

I can’t say for certain but most electrical issues such as you are describing are due to improper grounding.
Most of the Haynes manuals have good wiring diagrams to follow. It is tedious but I went through something
very similar on my GTA. Good luck

You might check the hazard warning flasher.

Thank you guy's for replying.

Replaced both Hazard and turn signal flashers. Still doing the same thing. I'll start with the grounding. I just know it'll be something simple ,like a frayed wire or no ground at all.

All rear bulbs have been replaced.

Some of the wiring does feel brittle. Oh well, it IS a 40 year old car !

Thanks again !