I bought a 99 mk4 Jetta a few months ago and it came with the problem. I haven't had the chance to get my hands on it and now I want to try to fix it.
The fuel door mechanism is stuck in the open position, fortunately it is not in the closed position, and the trunk lock only opens if I use the key directly. The actuator on the trunk lock is working since it does actually operate when I use the key, or if I remove the liner and manually move the switch that is operated by the key then the actuator works.
The trunk and fuel door buttons on the passenger door have no response at all. It makes no difference what position the valet lock is. For some reason only the trunk button lights up when the car lights are turned on, the fuel door button never shows a light.
The remote works fine for locking/unlocking the doors and for panic, but nothing happens when I press the trunk button, no sounds nor action at all.
Since the trunk actuator works it seems the signal is just not coming to it. I guess it is not coming out of the computer or there is an open wire somewhere. When I use the trunk button on the remote I don't hear any relay sound coming from the computer module (behind steering wheel) as it does when I lock or unlock the doors.
It has to be an electrical problem, I just don’t know where to start checking and I hope someone can suggest where to begin, or hopefully someone has had the same issue before and knows how to fix it.