I just upgraded my truck to 12volt. No wiring changes were made as I used the reproduction 6volt that I bought several years ago. I replaced all wiring from headlights to taillights. I replaced all 6volt bulbs with 12v. Now every time I push brake pedal there is no brake lights and when I push brake with headlights on the headlights dim a bit and the dash lights go out and the brake/tail lights go out but have a faint flicker.

What the heck happened?

 

To me it sounds like a bad ground issue. I'd make sure that there is a good ground to the taillights. Bad grounds can cause all sorts of weird issues.

 

The two things I would look at firs are grounds (if things act weird it is usually a ground problem). If they are OK I would check for shorts in the Brake power supply wire. When you changed from 6V to 12V did you change from positive to negative ground?

 

Yes... I did change from pos to neg ground.

I've had the truck on the road for 13yrs and have put probably 5-6 brake light "switches" in the truck. They seem to fail regularly for some reason.

 

Buy one at a Ford dealer.

 

Brake lamp switch is pressure activated, it's notorious for getting gummed up with dirty brake fluid.

C1AZ-13480-A (replaced 0A-13480) .. Brake Lamp Switch (Motorcraft SW-24).

1940/66 myriad FoMoCo vehicles.

 

I will also get a new brake light switch.... I'll get a good one this time instead of an O'Reilly special