Electrical problem with a Z3 ...

The car runs great as long as it's driven regularly.
The alternator recharges the battery with no problems but if the car is stored for months at a time the battery drains completely.

Alternator has been replaced and two new batteries since.

This all started happening around the time the passenger seat motor was fixed. I suspect it's related? Anyone able to offer advice for tests that would determine the cause?

It'll be time consuming but you should try to turn the car off and have it the state that its in when you store it. Then take a current reading off the battery to see how much power its drawing in that state. I would try using a volt meter and touching the positive terminal of the battery with the red end of the voltmeter and the black to a part of the chassis. This will tell you the votage that is being drawn - if you select amps then it will give you the amperage.

Now the things that I think will be drawing power are: the radio (possibly) and the clock. I would try to find the fuses for those things and pull them out (don't get them confused because they may have different ratings). Check the amperage/voltage now. If you have a current still then keep looking. You can easily isolate components using the fuse removal trick. Keep doing it until there is no more current being drawn.

The last fuse that you pulled will probably be the one that has the circuit that's leaking. The problem will be is the leak is somewhere in the chassis and is a short or something similar, you may not find it then.

The other thing to do is to disconnect your battery when you park it for a long while. This is probably not a bad idea anyway. If you battery has no charge when you plug it back in after a month or so then you need a new battery because the short is internal to the battery.

I hope this helps.

Electrical problem with a Z3 ...

The car runs great as long as it's driven regularly.
The alternator recharges the battery with no problems but if the car is stored for months at a time the battery drains completely.

Alternator has been replaced and two new batteries since.

This all started happening around the time the passenger seat motor was fixed. I suspect it's related? Anyone able to offer advice for tests that would determine the cause?

I just had my battery go flat. The solution to my problem, however, was simple - close the trunk lid so the light would go out.

Your problem is not as simple. If you have an inductive amp meter you don't have to disconnect the battery cable to determine if something is drawing down on the battery - just attach it to the cable. If you have a regular amp meter, you'll have to disconnect the battery cable and insert the meter in between the battery post and the battery cable. You should get a reading because.... your truck is open and the light is on - my problem. After removing the light bulb you normally would expect the amp meter to show no current flow, however, given your situation, you would expect to see the life being drained from your battery. If this is the case, ( the amp meter registers current flow) start pulling fuses(fuse box is in the engine compartment -left side) until you have isolated the circuit. If it is the circuit providing power to the power seats, faulty wiring to the seats could have been the cause of the power seats problem in the first place.

You may also want to look into getting a float charger if you leave your car sit for long periods of time. I hope your battery survived the ordeal. Good luck.