Lighting Problems

svt_gEEk

New Member
Oct 19, 2009
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Hi, I am a new member here. My wife has a 1999 Mustang GT 5-speed and we are having some lighting problems with it. The fog lights do not work, the indicating light on the switch doesn't even come on when you try to turn them on. The 3rd brake light on the trunk lid doesn't work and the reverse lamps do not work either. None of these have worked for as long as she has owned the car ( about 2 years ) but these problems really bug me and I want them fixed. I'm a fairly good electrician/mechanic so I can fix them but I am at a loss as to what the problem is. I figured it was probably a fuse or the lights were burned out or something simple like that but it isn't any of those, fuses and bulbs are good. I don't have a wiring diagram for the mustang like I do my SVT Focus so I'm not sure what components are on the same circuit as these. Does the fog light have a high-beam cut-off relay? If so, that would be something else I could check for the fogs, but the reverse lamps and 3rd brake light problem makes no sense to me. Any input?
 
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Wiring diagrams

The wiring diagrams I have access to are for a 2003 model year. Not sure if there are any differences that may affect your problem.

I agree that your problems don't have an obvious common cause other than the Multi-function switch and main power. Has there been any work done that may be important (audio install)?

Note, I have not had time to really look over the diagrams. I thought it best to give you the information I could.

Look over the diagrams and let me know if there are any specfic questions.
 
The multifuntion switch isn't even a common cause between them because the brake light and reverse lamps have nothing to do with that. The only common point between them all that I know of is main power.
 
The fog lights do cut off when you switch on the high beams. They are connected to an underhood 20A fuse labeled DRL FOG HORNS and they also have a separate relay. I had a problem with my reverse lights as well, it turned out to be the connector coming off the transmission for the reverse lights, or at least that's what the Ford Tech told me when they fixed it under warranty. So you may wanna check the transmission range sensor or the backup lamp switch. As far as your 3rd brake light, there is I believe a red/light green wire coming off the brake switch that connects it. Check the wiring or even the brake switch. I HIGHLY recommend purchasing a HAYNES manual or a subscription to ALL DATA.com. The HAYNES manual would be cheaper overall and I think is more useful due to the fact that you can take it with you and you don't have to print the pages out when working on the car in the garage. There are some awesome wiring diagrams in there as well. Also you can go to AUTOZONE.com and put in your vehicle info, then go to repair info and then select repair guides, then take about 2 minutes to register for FREE and then you can look at wiring diagrams on there, but they don't seem to be as detailed. Hope this helps you out.
 
Ok, so I had already decided to check the fog lamp relay just to see if that's the problem for that, I'm hoping it is because I have no idea what else it could be if the switch is good, the fuse is good, and the bulbs are good, the relay just about has to be the problem.

Thanks for the tip on the reverse transmission range sensor, I'll check that one out. How do I tell if it is good or bad?

As for the brake lights I have no clue. The only brake light that doesn't work is that 3rd light on the trunk lid. Since the others work I'm assuming the switch is good, must be in a fuse or wiring somewhere.
 
Wiring diagrams will be needed

I can not find anything that all 3 systems have in common besides they don't work.

I would start first at the fog light switch. Verify 12V power #16 BN wire (head lights on). If no power, problem is UPSTREAM.

If power, verify 12V on the #16 WH/BK. If power, then either the Ground G204 is bad or the indicator light is bad.

For suspect bad ground G204, measure Ohms between #18 BK and battery neg (everything off).

After this, move on to the Fog Lamp relay and report back.

It may be likely that the other problems mentioned have nothing to do with each other. Recommend diagnose each separately.

Consider buying a Ford service CD off ebay. You are going to need the detailed wiring diagrams.
 
There's a generic electronic module under the left side of the dash. It looks like those circuits run through that. Might want to check there.

BLK/PNK is for your back up lights.

BRN is fog lights and most of your other lights so probably third brake light too so I don't think the problem will be in the module for the brown line. There is a main harness in the trunk back by the fuel cut off switch. You might want to check that harness.
 
The back up lights don't go THROUGH the GEM. There is a tap in the circuit that lets the GEM know when the circuit is active.

Same thing for the brake lights. In fact, the 3rd brake like taps the feed just after the brake switch. Very simple circuit. Odd since the rear brake lights do work.

I tried to check for common grounds and splices. None that I could find. If this has a common cause, it's going to be some major connector within the body. Otherwise, I am out of easy ideas. Some additional clues would be nice.