Burkekernen

I bought an El cheapo center brake light. I hooked it up and with the ignition on but the car not running it works fine. But if the car is running it doesn't work and I get a dash warning. Anyone know what it is looking for from the light? A certain resistance? I know people will say just buy the $200 one but I'm curious as to what the car is testing for when it is running and why it doesn't do this with the engine off.

Burkekernen

Turns out to be resistance. Or more specifically it measures the current draw. The stock light pulls 100mA and my new one pulled 260mA. Not quite sure why they care if you pull to much as that means the light is working as opposed to no current which means it is burned out. I put a resistor in line and now my current draw is 50mA. Lower than I wanted but it worked and no brake light out message any more.