2005 Cadillac Deville dies when any light is turned on

Look for good motor to chassis ground or obvious wiring problems ,

Sounds like a bad ignition system ground. Do you have any lights, that aren't turned on, glowing dimly? Anything on the vehicle that's not working or not working correctly? Check those circuits first. If you have a bad ground electricity will "hunt" for a ground and cause lights to glow dimly, etc. Also, if you have an aftermarket stereo, check the wiring for that too. They're often the source of electrical problems. Hope that helps! Jim

Check the battery cables and the power distribution box. It is a long run from under the rear seat cushion to the engine compartment so you could have a problem almost anywhere.

When the ignition is on, the mirror turn signals are lit up. Other than that, all lights seem to be normal. When I pull the park lamp relay, I can turn the lights on, but blinkers will still stall the engine

Another fun fact. Disconnected head lights, and when I turn the headlights on and use the blinker with key off, instrument cluster lights up like the key was switched on

I guess my biggest question is "so don't replace BCM?"

It really doesn’t sound like a BCM problem, it won’t cause the engine to die out. Besides I doubt you can find one right now, I’ve been waiting for over 12 weeks for a BCM and it still shows on national back order. If you really think it’s a BCM problem you might consider a reflash.

Honestly, I'm with you on it not being a BCM issue and it being a ground issue. I'm assuming the headlights are in the CAN system because with both headlights disconnected, engine won't crank. Sorry for all of the questions. I'm a diesel mechanic, wiring issues are a lot easier with Diesel does anyone have a wiring diagram?

If you can’t find one on the internet then try your local library in the reference section and see if they carry the vehicles shop manual, if they do have the wiring sections copied from the book.

So I guess my next question is, am I looking for an improper ground or a short to ground issue

The way things are acting I would say an improper ground. Now these Cadillacs are ground crazy and they all must be good or you get feedback like what your experiencing. After we got rid of out last Deville we decided no more Cadillacs.

So I found the issue. It was a ground distribution block by the front right headlight. It wasn't bolted down lights work fine now. Thanks for everyone's help and advice