So I bought an 86 Chevy k20 Silverado 4x4. It had a 305 originally and was swapped out with a 454. When I purchased it, it needed a LOT of love and finally got it running smoothly. I now found that my turn signals don't work properly.
When I turn on the left or the right signal, both the left and right rear lights go on. The front works perfectly in both left and right and emergency signals (now that I replaced the blown bulbs.

I looked on all the forums and learned that I need to:
1) Make sure all the bulbs are the correct bulbs. (done)
2) Check Wiring. I'm not an electrical guru, but I did find that the dark green wire wasn't supplying any power to one of the rear passenger bulbs. I ran a replacement wire from the matching contact in the Direction/Switch panel attached to the firewall and got that bulb to light up. But they still ALL light up. I have checked the grounds and they all seem good as well.
Symptoms I have caught:
- Emergency lights work fine.
- Side marker and front lights work fine.
- Dash turn signal indicator lights work fine.
- If I hit the brakes when the emergency or turn signal lights are one, all signals go on and the indicator lights also stay on.

The only issue I have found (and I don't know if it is an issue) is the brown wire coming off the Direction/Switch panel has no power. I'm not sure if it should or not.

I am totally stumped. Any help would be MASSIVELY appreciated.

 

I'm also having a similar issue with my left blinker. when i flick it on my brake lights flash. I replaced the panels that fit on the light covers and i still get the same problem. I haven't replaced the harness yet though. that will be my next attempt at fixing the problem.
good luck.

 

Sounds like possible chassis ground issue. Check the one on the right side of the transmission bell housing first. It's usually the first to go. My truck has 5 chassis grounds if I remember right.

 

I had no idea there was one there! I'll look into that. Thanks for the tip!