2000 Ford Mustang GT. High beams BOTH WORK. Signal lights BOTH WORK. Low beam ONLY works on the right side. Both bulbs have been replaced. What is going on???? This is driving me mad!!

" dim headlights "? None of your lights should be 'dim'. You have 4 lights.

Side Lights.
Daytime Running Lights (DRL)
Low Beam Headlights.
High Beam Headlights.

Here are the fuses, check fuse 5 and D although they covers more items than lights. Your best bet is to find a good local auto-electrician.

1994-1998 Ford Mustang Fuse Box Diagram

https://knigaproavto.ru > ... > Ford > Mustang

17 Oct 2019 - Fuse Panel Diagram Ford Mustang Mk.4 engine: 3.8L, 4.6L, 4.9L, 5.0L and 5.8L (1994, 1995, 1996, 1997, 1998)

More here

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Check the wiring on that light. Also check the bulb with a couple of wires to the battery, or swap the bulb left to right to make sure the new bulb is okay.

If this fuse box diagram below is correct you have a fuse for left low beam head light and another fuse for right low beam head light and a third fuse for both the high beam headlights. If the fuse is good for the right I think I would take the left low beam head light and put on the right side and see if it works on the right side. If you think you got the wiring correct. It may be the bulb is bad from the wires which shorted out. A voltmeter or test light to the socket would save some time switching the lights. I have had the socket bad before and had to change the bulb socket.
https://fusesdiagram.com/ford/fuse-box-diagram-ford-taurus-2000-2007.html

Hi, check you bubble or replace it.

Do the high beams work?

The ground point is on the apron ( the inside of the fender, just behind the headlight). There is one on each side. From the wiring diagram, there is just a splice connecting power to the lights in parallel for the high and low beams, but the grounds are separate.

If the RH high beam and the LH low beam works, then it is not a ground problem. You could just tap the LH wires to power the RH, but I think that if you look closely the connector for the bulb is probably melted. If that is the case then you will have to splice on a new connector.

The colors are as follows:

RH --> low beam power ( light blue)
LH --> low beam power (white OR light blue)
BOTH high beams --> ( red/white stripe)
Ground --> Black

Sorry, The diagram is too large a file to upload.

If you have separate bulbs for the low and high beams(4 light system), if you pull the turn signal and hold it, all 4 lights will stay on until you release the turn signal. You need to replace the bright bulb on the right side and the low bulb on the left side. If your headlights only have one bulb on each side, then you will have to replace both of them to get everything to work.

its probally the bulb........alot of car headlights are DUAL element meaning one bulb has 2 little elements inside one for low beam one for high beam...............Most times the low beam burns out because its used more.....get it one bulb gives both high and low beams.......REPLACE bulb......if you don't know what kind go to auto part store, tell them the make of your car and they will give you the right one .....

Remove the plastic trim from around steering column so you can access the wires to the multi-function switch(turn signal switch).The RD/YE wire is the feed from the headlamp switch for low beams,it then leaves the multi-function switch on the RD/BK wire and feeds fuses no. 10 and 4 in the under dash fuse panel and from there to the headlamps right side is the DB/LG and left side is the WH/LG. Get you a 12V test light and make sure you have voltage in these places. I suspect that the multi-function switch itself has gone bad since you stated that they would work at times when you activated the turn signals.

No as a rule they don't fail at the same time, in fact it is unusual for the hi beam to fail ever, the problem is most likely in the turn signal/multifunction switch, this does cause your problem, have seen it many times.

check the fuses under the hood these headlamps are seperatly fused,
later
paul