Weird Lighting Issue

[Jon87V20]

Hello, everyone. I've been experiencing a weird issue with the exterior lighting on my 87 Suburban V20. When the brake pedal is pressed, the running lights and dash lights come on dimly with the brake lights. If I turn the left turn signal on, this stops the problem. Also, if I have the headlights on (or running lights), the left turn signal will stay lit, but will not flash.

I tried changing the headlight switch yesterday, which hasn't helped. I noticed this morning while driving with the headlights on, my trailer brake controller's power light is fluctuating brightness and sometimes goes out. It normally only lights when the brake is pressed.

Any ideas on where I should start? Is this just something after the brake light switch shorting in to the lighting circuit?

Thanks!

[bd]

It sounds like a floating ground.  Check, clean, service ALL of the vehicle grounds.  Disassemble them, burnish the connections to shiny bare metal and reassemble with a coat of antioxidant paste.  Verify and service the following ground inventory:  battery-to-engine, battery-to-radiator support, battery-to-right frame rail, rear of right cylinder head-to-firewall sheetmetal, instrument panel wiring-to-kick panel under the dash next to the emergency brake pedal assembly, tail/brake lamps-to-bed, front turn/park lights-to-radiator support, and headlamps-to-radiator support.  In addition, check the tail & turn lamp sockets for corrosion and clean or replace as needed. 

[Jon87V20]

Ok, will do. Thanks, bd!

[Jon87V20]

Update: Turns out it was something shorting up behind the bumper with the old trailer light harness. Seems to be back to normal now.

[bd]

Glad you got it sorted out.  Be sure that any repairs you make are weather sealed so moisture and road salts can't intrude the wiring.