Third brake Light

WAYNE PFEIFFER

MEMBER
2012 FORD MUSTANG
3.7L
V6
2WD
AUTOMATIC
75,000 MILES

Third brake light was not lighting, replaced third brake light and BCM. Worked for a few weeks and now no third brake light again. Checked third brake light with battery and lights fine. No output from BCM to third brake light. All other lights working fine. Traced wires and cannot see any breaks or worn down insulation, I am at a loss.

3 Replies

HMAC300

MECHANIC
48,601 POSTS

Check the two wires marked coming from multi function switch that go to body module see if they work with brake applied. Then check third brake light ground center of trunk.

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Sunday, June 25th, 2017 AT 10:17 AM

WAYNE PFEIFFER

MEMBER
2 POSTS

http://iihs.net/fsm/?d=711&f=090%20-%20Turn%20Signal%20and%20Stop%20and%20Hazard%20Lamps.pdf&p=5

Here is what I am getting turn signals work fine, it is just the third brake light not working. The ground for the third brake light is not on the trunk, it joins with the trunk release and right brake light to a common ground with other circuits. Checked the ground and it seems to be fine. I am getting the brake switch signal into the BCM, but not the signal out to the third brake. all the other brake lights are working fine which uses the same brake switch signal and all run through the BCM. The third brake light is working fine as I use a battery to power it and works fine. I have checked the wiring for pinched wires and rubbing but cannot find anything. It will work for a few weeks then for some reason, the BCM will blow again. I am just at a loss on this one and just looking for suggestions/ideas, heat related? known problems with Ford BCM's?, updates? Thought someone would find something I am missing.

http://iihs.net/fsm/?d=711&f=090%20-%20Turn%20Signal%20and%20Stop%20and%20Hazard%20Lamps.pdf&p=4