Passenger side taillight not working -2001 model

[TheRedGlacier]

Hi guys, I posted this awhile ago, but we couldn't figure out the issue. I've got a 2001 A4. The passenger side lower taillight does not work as a running light. It will work when the brake is pressed, but no other time. Correct bulbs are being used, and swapping bulbs made no difference. I cannot see any visible corrosion around the connections, but does anybody have any ideas? I've had the car since September, and just put it back on the road (my summer car's distributor is dead). In the past I ran the fog lights at night so I could have the secondary (top) taillight bulb running, to avoid being pulled over, however the fogs are very blinding.. Does anybody have any suggestions?

[audinutt]

Check for power at the socket and check the grounds. Also I have seen the wrong bulbs cause issues.

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[TheRedGlacier]

How would I be able to check for power? Multimeter? I have 7528s in both "stop light" sockets. How would I know if the grounds are okay?

I'm sorry if I'm asking stupid questions, I'm not too familiar with electrical components.

[TheRedGlacier]

UPDATE: I have swapped the bulb carriers between wiring harnesses.. I moved the passenger side carrier to the driver's side, and the bottom taillight was running, so it appears to be something with the wiring harness on the passenger side. Unless... Is it possible that my taillight is not working, as my front passenger side parking light is dead? I know this may sound like its a long shot, but I have been told these cars are very finnicky, the signal light works, but the parking light filament is burned out, and I have yet to replace it. This issue started around the same time I noticed that the parking light was out.

However.. The rear taillight used to come on if you wiggled the bulb carrier, but that was also when the parking light wasn't shot.