Hi, joined AZ to see if anyone has suggestions ...

The 3rd brake light on my daughter's 2008 TL does not turn on when brake pedal is pressed AND engine is on. It does work properly when when engine is off and/or when keys turned to ACC ON position. All LED's in 3rd brake light are fully off when this problem occurs, i.e. it's not a "partially dim" or "a few LED's are off" or "flickers" or "sometimes happens some times doesn't".

I've replaced the brake light switch which seemed to be a magic cure-all for problems with third brake light, but that did not help.

Any other suggestions or pointers for a basic DIY'er?

 

The third brake light goes to a ground point after the light. If the ground is bad in any sort of way, it'll prevent the light from turning on. Sounds like the ground has some power going to it with the car on, but not with the car off. The ground is G501 according to the wiring diagram: