Why won't my BMW 328i lock and why is the battery draining? What switch or relay might be malfunctioning?

Question

On two occasions now (both when it's been very cold), my remote has refused to lock the car. I do get 3 chirps to indicate it hasn't locked, which tells me the remote battery is fine. (Usually 3 chirps means there is a door/hood/trunk ajar.)

Last time this happened, the battery slowly drained as well. The problem turned out to be that the hood contact switch was stuck--an easy fix. Now I'm having the same problem again (doors won't lock w/remote, getting the 3 chirps) but the hood switch seems to be fine. I didn't notice the problem until I opened the trunk (for the second time) today; when I closed it and then tried to lock the car with the remote, it wouldn't lock. I pressed the trunk switch several times and it seems to be working properly.

I'm thinking a switch or relay must be malfunctioning somewhere...any ideas of what I could test next, or where the problem might be?

Thanks Andy W for the research and links. The locks don't move at all, nor does the car arm when I press the remote lock. It tries to arm, but can't for some reason (thus the 3 chirps). I took it to the shop last night to see what they could find today, so I'll point them in the direction of the trunk wiring. From the forum links you posted, it sounds like trunk wiring has been a big issue in the E36. My car is a '98 so I imagine there's a good possibility that the wires are damaged. I'm also curious to see if the battery drained at all overnight...I was assuming it would drain like last time when the hood switch was faulty, but it started all day yesterday without any trouble so maybe that won't be an issue. You know, this car has been great up until about the past year when everything has started to go awry it seems...

Answer 1

do the locks lock, then pop up, or do they not move at all?
ive been searching my bmw forums, and it seems that there are two causes of this.
if they pop down then pop back up, its due to a faulty central locking unit.
however, when the car arms, but doesnt lock, this has to do with bad wiring in the trunk of an e36. It causes a short, and the central locking cant read that the a signal was sent.
http://forums.bimmerforums.com/forum/showthread.php?t=516269&referrerid=&highlight=trunk+wiring+problem

heres an overview for the fix
http://forums.bimmerforums.com/forum/showthread.php?t=79896&referrerid=&highlight=trunk+wiring+problem

Answer 2

have the alternator checked, it might be getting weak