Rear tail problem, or haunted car?

[Cveezy]

Hello, I have an e24 1985 635csi. My rear passenger tail light is not working. The rear driver's side works 100%. I'll be separating info to make it easier to explain, as well as troubleshooting done.

What works
Reverse light
Brake light
Turn signal

What Doesnt work
Rear tail light (light that glows so people know you are there ill call it "idle light")

Vin= Voltage in

1)The first thing I did was swap bulbs, left to right and right to left. The known "good" bulb did not work on the passenger while the "bad" bulb worked on the drivers. So the bulb is not the problem.
2)The socket was cleaned to make sure it was not a contact problem. It did not resolve the issue.
3)Problem should not be the fuse as the driver's side works and every other light in the tail light works.
4)Took a multimeter and checked at the socket voltage.. was 0
5)Removed "wiring harness" from back of light, checked voltage at idle light Vin and ground directly at harness. was 12V (great this means the problem is the socket.) Wrong
6)With loose wires attached light bulb to the harness...no light. While still attached to bulb I checked with multi meter, and had full 12v...(WTF!?)
7)Took wire out of "idle light" Vin and attached to turn signal and it works (I had the signal blinking for testing purposes)
8)I then ran a Vin line from the plate light to test the ground wire and there was no problem.

I'm lost. The car clearly defies laws of science in my step 4,5,6. Anyone have some helpful insight,hints,prayers,voodoo?

[alpinacsi]

So the issue is between the connector to the taillight and the bulb? Have you checked the path thru the light assembly? Is the light assembly connecting properly to the harness?

[dmac156]

Clearly haunted! Sounds like you are doing proper troubleshooting. It could be the harness that runs from the driver side to the passenger side, or a ground on the passenger side.

I would remove the passenger taillight assembly and plug it in on the drivers side making sure it's grounded. If that works then it's almost certainly in the wire harness. If it does not work it is most likely in taillight assy.

[Cveezy]

So the issue is between the connector to the taillight and the bulb? Have you checked the path thru the light assembly? Is the light assembly connecting properly to the harness?

1)I believe I proved this was not the issue in test 6&7 as even when i directly connected the bulb to the harness it did not work.
2)the grounds for the idle light and turn signal are connected, and the turn signal works fine so that does seem to be a problem. as for the voltage in, 6&7 tested that.
3)Everything seems to be connecting correctly, everything else works when connected.

I appreciate the thoughts though. Makes me rethink how and what i tested

[RSheiman]

One of my fogs did not work yet there was power. I realized that when you use a voltmeter you usually ground it to whatever is around you. Well turned out my ground to the fog light was out so I just ran a new one. I don't have the wire diagram in front of me...but did you ensure proper ground for the light?

[carnut1947]

Run a new ground wire

[SprCpe]

Had a similar problem with the 89, right side, turned out it was the ground to the socket itself if I remember right, I think each socket has its own ground :-(

[Cveezy]

Had a similar problem with the 89, right side, turned out it was the ground to the socket itself if I remember right, I think each socket has its own ground :-(
Actually in my tail light all sockets are daisy chained.

Turns out there was a loose connection in the relay in the back! Its all good now.