Yesterday I noticed that my driver's side stop light was out on my 2001 A4 1.8T, so I bought a set of replacement bulbs from wal-mart, and replaced the bulb without a problem. However as soon as I turned the car on, I noticed the passenger side stop light was now out. Being that I had bought a two pack of bulbs, I immediately went to replace the passenger side stoplight. After taking out that plastic light bulb backing that holds all of the bulbs in place in the lens, i noticed that there was not even a stop light bulb present. This was strange, as i had always had a functioning passenger stop light, but I figured the bulb had come loose and is now floating around somewhere in the light lens. However when I put the new bulb in and went to replace the apparatus that holds the lights in the lens, I noticed there were circular holes for the tail, reverse, and turn signal lights to go through, whereas the passenger stop light had a small rectangular opening. The same bulb that worked as a replacement on my drivers side would not fit into the passenger side tail light. Does anybody have any idea why this is, or what I should do? Much appreciated.

 

That is the spot for your rear fog bro. One of them is square because it is only meant to be on one side. The control is on your dash.

 

So it is. But that raises further questions. The driver side configuration is for four round bulbs (P21w, i believe). Assuming the bottom three are the same as the passenger's side, it appears correct, but what is the top bulb for on the driver's side, and why did replacing it eliminate the driver's side stop light problem? Upon further investigation of the bulbs themselves, it appears that the bottom most bulb functions as both tail light and stop light (on both sides?), as it is the only bulb with two filaments. But... both of my tail lights work. Could it be that one of the filaments is bad on the pass. side?

Basically this is what my post boils down to: Do I need to find a bulb with two filaments to fix my passenger side stop light, and what is the deal with the top bulb that I originally replaced?

 

There are 4 bulbs on the drivers side and only 3 on the right. At least on my 99.5 i believe previous ones should be similar. The top one is the rear fog light and the right side is blocked because it doesn't take a bulb.

 

I don't really understand your question but the bulb for the brake light should have two fillaments as it stands for brake light plus tail light.

The most upper one on the driver's side (rear fog light) should also be two fillaments which explain why they are so bright.

Hope that helps.

 

I got it now. I misunderstood the first answer I got. I thought you were saying that the fog light bulb occupied the rectangular opening on the pass. side. I have another question about the license plate lights, but I'll make another topic.