Something went wrong electrically on my 96 max. When you switch the running lights on, the brake lights have started to come on as well and they stay on with the running lights. With running lights swittched on, when you hit the brakes, all five lights (two running and three brake lights) brighten a bit. Release the brakes and these five lights just dim slightly. Again, his thappens with only the running lights switched on.

How would you begin to diagnose a problem like this? Any advice appreciated.

 

Are the wiring harnesses exposed in the trunk (or anywhere else) so that something could rub until shorting the two separate positive leads together? Or pinch them together?

 

check all your bulbs, if you recently changed any bulbs on the car...., you might have placed a single filament bulb in a spot where it requires a dual filament. Or you could have a dual filament bulb that's gone bad and is touching.

example of dual filament bulb location would be a bulb used for both parking and brake, or parking and signal, if you put a single filament bulb in, anytime you step on the brakes, you power up the parking lights and vice versa....if you turn on the parking lights, all your brake lights will light up

 

I'll check the trunk wires again. The problem developed without any bulb being touched, except the courtesy light in the passenger door. I wonder if a dual element bulb in the running lights had one filiment shorted? Worth opening them up to take a look I guess.

Thanks for these suggestions. Any other ideas would be welcome.

 

A guy who knows some stuff about cars here in Seattle thinks the trouble is in the BCM, (Body Control Module). We are going to remove it and have a look. My question is: Has anyone been able to see diagnose a problem with a BCM and fix it? Or is a replacement the only way to go?

 

Are you sure they only stay on when the running lights are on?
Usually the brake lights stay on when the little rubber stoppers on the brake pedal fall off and need replacing.

 

Yes, hitting the brakes just makes all five rear lights brighter. If the running lights are switched on, the fuse blows and there are no lights at all. The car is unsafe to drive without them.

 

The BCM was fine. The turn signal multi switch was fine as well.