True brake light mystery...a puzzle for everyone..Please help me

If the fuse was missing then he had to pull it out meaning he was having probs with the brake lights and took the easy way out.

You say he removed the spoiler?? As in for good? In that case what did he do with the 3rd brake wiring?


I'd do a visual inspection of the wiring around the hatch area (esp the 3rd brake light wiring) and see if anything jumps out at you first. Then i'd start to go to town with a mutlimeter.

I have the wiring diagrams for an 1988 Mustang handy.

According to my diagrams, the brake light HOT wire is a Red wire with a Light Green tracer from the brake switch all the way to the lights themselves. It passes through three connections : C219, C216 and C405 and the 3rd brake light can be disconnected through C453

However depending on if your car is a stick, auto, coupe vert or hatch, the wiring is slightl different. I'd check C453 first (if checking the 3rd brake light wiring doesn't pan out). It's located above the LH rear wheel well in the interior right above where the shock bolts. There are three plugs there. 1of them is an 8-wire black connector and another is a 2-wire black connector. The 2-wire goes to the 3rd brake light. Unplug it, pop your fuse in and hit the pedal and see if it holds. If it does, the prob is in the 3rd brake wiring. if it pop, the prob is in the main taillights.

Then disconnect C405 which is the 8-wire connector. This will kill power to all the brake lights. Pop the fuse in and hit the pedal and see if it blows. If it does, the pinch is somewhere in the body wiring. Connector C216 is by your left foot and can be checked as well if needed

Red with light green tracer is your wire from the switch back.



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bubba-dough said:

Boy that is some bad troubleshooting practice! Please DO NOT pop fuses in to find shorts. This is absolutely the WRONG way. There are very easy ways to find shorts for free (fuses cost money, a hassle, and this way can cause other potential failures).

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Ok, Crack out the DMM and substitute popping in fuses for checking the resistance of wires while unplugging the connections and use that method instead. You are right i shouldn't have described such a hack method. It makes me look bad.

He still needs to do a visual inspection beforehand anyway and check he spoiler wiring like i suggested. If he doesn't find anything they he'll have to start opening up the connectors and test the actual wiring.

So instead of popping in fuses, I'll retract and say just use your DMM instead to check resistance values to ground.


PS: I was referring to reading electrical diagrams when i made that comment

the brakes were always fine until he painted the car, and when he painted it he removed the spoiler. Could this be connected or just a coincidence? is it worth is to pull of the spoiler?

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PSS: I think his prob is in his spoiler wiring, The PO prob pinched the wiring when he put the LX wing back on the hatch since the wiring lays loose. I bet if he yanks off the wing (easy to do BTW) he'll find the wiring pinched between the spoiler and the hatch. He may be able to tape it up or he'll have to cut and splice the wiring to repair it, but i think the most he will have to do is yank the spoiler off, find the pinched wires, re-splice and heat shrink to repair them and carefully put the spoiler back on and that's the end of it. It's always the most logical thing.

I wouldn't even test the connectors. First thing i'd do is pull the spoiler off. It's very simple. I think they are 8mm nuts but not sure.

Well that does change things since the stop light circuit shares some of the wiring of the hazard circuit.


I'll post up later with more connections to check

Which wire was getting hot? The Red green stripe wire??


You can try disconnecting it on the driver's footwell. The connection is C216. Remove the plastic kick panel and look for the main "thick: harness. There will be two 8-pin connections there. One will have a black plug, and the other a grey plug. The grey plug will have the stop light wiring in it. You should see the red green stripe wire in the bundle there.

Unplug it, and test the wiring. See if it clears up the short and hot wires at the MFS on the column.

If it does, you know the problem is further down towards the taillights. Might be time to pull the interior panel off and open those connections.

Still curious to why you haven't pulled the spoiler off yet. It's a lot easier than yanking half your interior out to get at the two plugs by the rear shock.