With the light swith off, I press on the brake pedal and the dash lights come on and the door chime sounds, and the tailghts (not Brake lights) come on as the brake lights would. With the light switch on, the brake lights are on like the normal taillights should be, and nothing happens when I press brake pedal. I've changed brake light switch, ignition starter switch, ignition lock cylinder, checked fuses and nothing has changed. Very confusing. Any suggestions on what it could be? Thought bad ground but all the other lights work fine. Any help or advice is greatly appreciated for I am stumped. Thanks

 

Also forgot to mention that the ABS light comes on sometimes while driving. Doesn't flash just stays on. But isnt on all the time

 

The Light Switcheroo, sounds like the bulb is reversed... !

The Brake Lights, Directionals, Flashers, all go through the Multifunction Switch mounted in the Steering Column; just might be something wrong with it and you are getting feedback through the system.

 

Thanks! I'll try that next. Expensive piece of plastic 😒 I'll send an update. Parts store had to order it of course

 

In 1993 the brake lights have no connection to the turn lights or multi-switch, so not multi-function switch issue

This is 1994 but sane as 1993 wiring and colors

 

After you down load and look at the diagram

Pull out fuse 10 in cab fuse box

See if the lights stop working, switch on/switch off, pedal/on pedal off, they should, if parking light wire is shorted to brake light wire

I would suspect someone "tried" to install some trailer light wiring and goofed, or wires have come apart and are shorting at the back

So the light green wire's brake light 12volts is passing over to the Brown wire(parking lights) some where, and not connected to the rest of the light green wire running to the brake bulbs so brake bulbs don't work, which is why I think this was done by a human, some one cut the light green wire

If you find a Black Box under the rear bumper then that may be bad, it is to combine brake and turn signals on 1 wire for trailers

There are only 4 wires you need to check in the rear
Light Green, for brake lights
Brown, for parking and license plate lights
These share 1 bulb, but is a TWO Filament bulb so separate inside the bulb

Light Green/orange stripe, Left turn, has its own bulb
Orange/light blue stripe, right turn, has its own bulb

I didn't include the Ground wires but they are there as well, but if lights seem normal brightness I doubt its a bad ground

 

None of the lights work with the fuse unplugged. The lights are working on rear, but with the light switch on the brake lights and tail lights are switched. And with the lights off, I hit the brake pedal and the chime sounds, brake lights come on but delayed and very dim. I replaced the multi function switch and light switch just to be sure and that didn't change anything which was expected lol. Almost like it's back feeding, from a bad ground. But I can't find a ground anywhere on rear.... License plate lamps are also being affected, very very dim.

 

Rear Lights on but dim means the ground in the back is GOOD, and another system in the front/cab is using that ground because its ground is bad

 

Figured it out! I'm an idiot, had a positive and negative wire connected at the rear where I removed a trailer plug harness... Respliced the wires correctly and everything works perfect. License plate lamps still don't work but that's not a big deal

 

Good work

Thanks for posting the fix

 

Thanks for the assistance and information, very much appreciated

 

Good you found out the problem, as you had said, "It acting like a bad ground" !

Something for others to watch out for.