Just checked voltage in sockets with parking lights on. Driver side had around 6 and passenger side has 0-0.1. The wiring looks fine, but i still dont understand how the turn signal and hazards work perfect on both but parking light doesnt on one side :/
Alright, here's the thing - and warning, this will be a lot of text, please take a minute to read it and digest as I think it will help you find your issue. I apologize for the wordiness but I wanted to make sure I was clear in my directions. (I added a picture at the end in case of the wall of text is just too much.) So onward...
The parking bulbs are dual filament. Which is also why there are 3 wires going into the socket. 1 wire is ground, 1 wire feeds voltage for the parking light, the 3rd wire feeds voltage for the turn signal/hazards. When you have the parking lights on, one of the filaments in the bulb lights up. When you flip on the signal or hazards, the second filament is pulsed, giving the flash. Since the parking lamps and turn signal/hazards are feed from two different circuits, this is how you can get a situation where the turn signals work, but parking lights do not. I took a look at the wiring diagram, so here's what I've got:
1) if you recently replaced the bulb, confirm that it is in fact a dual filament bulb. It should have two little spring-looking things inside of it (this is what gets hot and puts out the light), not one. Did the passenger bulb work on the driver side, for both parking lights and turn signal/hazard? If yes, then the bulb is good. If it didn't, or if it has only 1 filament, then you need a new bulb. I suspect your bulb is fine.
2) Assuming the bulb is good, the fact that the driver side bulb does not work properly in the passenger side leads to me think the same thing as you in your first post - there is something wrong with the wiring. So, taking a closer look at the wiring:
On your passenger side socket connector, there should be 3 wires present. The colors I will provide are based off a 95 Mustang wiring manual, so if you have different colors, please stop here and let me know. (Note: to measure at the connector, either stick your digital multimeter (DMM) probe into the back of the connector where the wire enters, or if it's too big, use a straightened out paper clip or equivalent and connect your probe to that.)
- One wire will be black - this is the ground. To test this wire, use your DMM to measure the resistance between the wire at the connection and a good vehicle ground (or negative battery terminal if you can reach it). You should see 0 Ohms for your resistance. If you don't, let me know. Since your turn signal works, I expect this to not be an issue.
- One wire will be Light Green with a White stripe - this is the + feed for the turn signal/hazard function. When you have the turn signal on, or hazards on, this wire will pulse +12v. Use your DMM to measure voltage here, you should see it pulsing something close to nothing, then close to 12v, then close to nothing, then 12v, etc. If you don't, let me know what you are seeing. You say that your turn signal works fine, so I expect this to not be an issue.
- One wire will be Brown - this is the + feed for the parking lamps, and the likely culprit. With the parking lights on, measure the voltage at the connector with your DMM, you should see a solid +12v. If you do see a solid +12v here, then the issue is with the socket itself. If you do not see a solid +12v, then the issue is with the wiring. Let me know what you find here.
Assuming, from the 3rd bullet, if you discover that there is no voltage at the wire, then there is an issue in the wiring itself. However, since your driver side works, that means that the issue is between the splice that feeds both passenger and driver lamps and the passenger socket itself. This part is going to be the b1tch part. To find the splice, start with the passenger side socket, and follow the Brown wire until it enters a harness. This harness should travel across the front of the car until it reaches close to where the battery is. From here it will split (what the factory manual calls a Take Out, or T/O), with some wires/harness continuing on to the other headlight/parking light, and the main trunk heading to the back of the car, parallel to the battery. It is at this split that I think the splice is located. Try to carefully unwrap the harness here, look for the Brown wire (but make sure it's the one heading to the passenger side, NOT the one heading to the driver side) and check again for voltage with the parking lights on. If you you do not see voltage, then move up the harness past the splice - move towards the back of the car, same Brown wire but AFTER it splits to go to the driver side lamps (meaning, check the voltage at the wire before it splits off to go to the different side lamps). Check voltage here - I fully expect you to have +12v here, as your driver side parking light works.
If you DO have voltage here, then the issue is that the Brown wire is cut or nicked between the split and where it terminates at the socket connector. Either unwrap the harness, locate the cut/nick and repair it, or maybe even easier is to just splice in new wire, run it along the harness, and connect it at the connector. It's up to you. Just keep in mind that if the wire was cut/nicked, try to see if there was a reason for it - as in maybe something is hitting the harness, so you can prevent future cuts/nicks.
If you DO NOT have voltage at the Brown wire BEFORE the splice, then either you are measuring voltage incorrectly/doing something wrong, OR I am wrong about the location of the splice. If you are thinking I am wrong about the location of the splice, here's what you can do. There is a black, 16 pin connector located near the battery (maybe under the tray?), that joins the headlight/lamp harness to the dash wiring. If you continue to follow the harness that the Brown wire is located in, you will eventually get to this connector. Look for the Brown wire (it's the only Brown wire going into this connector, located at Pin #4, which if you are looking at the female side of the connector, is located at the top left. Again check voltage here. You should, for 100% certain, get +12v here. If you DO NOT, you are doing something wrong. If you DO, then the issue is somewhere between this connector and the socket. Do NOT cut the wire here, as this wire feeds both the driver and passenger side. Work down towards the lights until you find the splice. Then do what I mentioned in the above paragraph.
Good luck, let me know if I can do anything to assist!
Edit: I made up a quick drawing which sort of summarizes what I am saying in words above (which you should still read).
Measure at each location: 1, 2 and 3, then let me know what you find.
