My brake lights work as long as the headlights are off. But once I turn the headlights on for night time driving, the brake lights both don't work. Scouring the electrical diagram but not seeing a common theme here yet. Any ideas?

I also have a weird electrical issue where the vehicle will actually not shut off when I remove the keys from the ignition if the headlights are on—soon as I turn off the headlights, the vehicle then turns off. Doesn't bother me since it only happens at night and it shuts off every time I switch off the headlights, but still, very weird... maybe a common thread.

 

Never seen that one!

As a diagnostic I would try swapping the headlight stalk switch, unless you can find something in the wiring diagrams that would suggest otherwise.

 

do your horn and CC still work?

 

jastutte, horn still works, cruise control works too but I have a vague memory of it not working with the headlights on... I never use it while driving at night however, so I can't confirm. I'll try headlights and cruise control next time I drive.

Abran, that's a comfort to hear even you haven't had this one. Headlight stalk switch is my first suspect too, I had to do the lithium grease trick a couple years back, but I'm going to spend a little more time in the electrical library and circuit diagrams before ordering a replacement. Hopefully something crosses over and it's not a BCU problem. Replaced the interior fuse box three years ago due to the corrosion issue, hoping the replacement isn't corroding too now...

 

Hmm. It does sound like fusebox corrosion (interior) to me.. is water getting in your dash / front seat area from something still?

The reason why I say this is .. circuits that are seemingly unrelated start to interact in weird ways. And the brake lamp relay is built into the interior fusebox (non removable)

 

Confirmed that cruise control doesn't work when lights are turned on.

nashvegas, I've not noticed water ingress to the footwell in this D2, but I do have a slight headliner leak from the gutter rail corrosion issue—haven't gotten around to fixing that yet. But never enough water to show up down low. Could be the used fusebox I got was already corroded and now is well on its way... It does appear from the electrical library that the IDM is a suspect as the headlights also run through the IDM.

Another interesting note from the library:
The cruise control ECU will engage the cruise control if:
The vehicle is travelling at over 28 mph (45 km/h)
The gearbox selector position is acceptable to the cruise control ECU
The cruise control switch is still on and has not been turned off
The brake pedal is not depressed.

Perhaps the system thinks the brake pedal IS depressed, due to a faulty signal from the brake pedal, which keeps cruise control from activating? Will report back as I get time to make more investigation into this.