W140 - Brake Lights Illuminate When Headlights Turned On
 

1996 S320, 135K miles... all three brake lights illuminate when the headlight switch is turned to full on, regardless of brake pedal position; with switch in off position, brake lights function normally. With headlights off, instrument cluster and odometer lighting dim everytime brake pedal is depressed; if pedal remains depressed for longer than a second, "Bulb Out" warning illuminates, although no bulbs are out. Warning light extinguishes and cluster lighting returns to normal when brake pedal is released.

Problem remains despite installation of headlight switch, brake pedal switch and lamp control unit (140 542 09 32) of known quality. Brake light illumination problems remains when wiring harness is disconnected from the instrument cluster.

Any ideas as to the source of the problem?

Clean and tighten any and all grounds you can find in the headlight and taillight area.

I second that.

Just curious.
was the car involved in an accident lately or had some repairs done that may have contributed to the problem?

Did you have any aftermarket equipment recently installed?

Vehicle is completely stock and accident-free. Checked and cleaned all accessible grounds but problem remains. Checked the wiring diagram... everything pertinent seems to route through the exterior lamp failure monitoring module (N7). Anyone have trouble with this unit?

Oh yeah, they can do funny things when faulty.