Ok have a 1990 GMC sierra. It is not really stock at all but the lights are. The problem is when I turn on the blinker it works fine but if I turn on the head lights or running lights they blink really slow. Now when I hit the brake with any of the other lights on they stop blinking completely.

The only thing I can think of is when I bought the truck the light mount for the left rear blinker/brake/reverse light was not working when I bought the truck so I took the mount off of my 1995 GMC sierra that does not run and put it on the 90. But as far as I know they are compatible.

So does anyone know what could be causing this?

 

Sounds like you got a bad ground somewhere.

 

Lol I am good with mechanics but not with electronics. How would I go about finding this to fix it and also is there any way to check to find out?

 

Check your ground wiring up by the headlights. Also make sure there is no corrosion.

 

ground is a good bet because with headlights on you remove the ground your blinkers backfeed through

 

He fixed it it was ground!

 

how did you find out i didn't see it posted, i am always interested in wiring problems

 

He had another post on here saying he fixed it and he and I Pmed back and forth. The guys who had it before him had a bad crimp on the ground wires.

 

Thanks for posting that I got it fixed. Since I was talking to you about it I forgot to post here that I fixed it.

 

No problem!~