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Checking the Battery Water Level

battery.jpg (61152 bytes)  The battery is located under the rear seat.

Every two or three months lift up the rear seat cushion and check the battery cell water levels.  If they are below the little tab visible through the hole, add water.  Preferably, use distilled water but you can use tap water in a pinch.   Do not overfill, the electrolyte will drain out and may damage the rear suspension.

An Issue with the Rear Power Distribution Box

rear_pwr_dist.jpg (59791 bytes)   The Rear Power Distribution Box is located under the left side of the rear seat.

My right-front door power lock stopped working one day after I checked the battery water.  After wiggling a few wires in this area it started working again.  I didn't find the problem but I did make a slight change.  This area of the car, although using fair quality electrical hardware, is a dog's breakfast when it comes to design.
The plastic box contains the ABS controller and the "General Module," the latter responsible for power doors locks and sunroof control.  The wires enter the modules from the underside.  On my car some of these wires were pinched under the foot of the distribution box.  I removed one 10mm plastic nut and pulled back two clips to lift the box far enough to move the wires clear of the foot.
If you decide to do this be sure to disconnect the battery to avoid any "accidents."

Alarm Module May Be Crushed By Rear Seat

After completing the above work I discovered a further problem.  The alarm module, mounted on top of the distribution box, contacts the underside of the rear seat.  On my car it had bent the enclosure and circuit board enough to crack the potting on the board.  You will need to remove it to determine if this is a problem.

alarm_locn.jpg (65800 bytes)

Relocate it to the right of the power box, attached by its edge to the lip on the seat support bulkhead.  The antenna connector is oriented to the upper left and the wire is bent towards the rear.  Drill two holes in the bulkhead lip to suit the mounting screws.  If you need to unplug the alarm module, pull the fuse out first.  Replace the fuse after reconnecting the module.

Recovering from Loss of Battery Power

If you disconnect the battery you will need to reset the radio code and the OBC time and date.  See your owner's manual for instructions.  When you do the radio code, please note that you need to actually turn the radio "on" first.

Auxiliary Fan Low Speed Resistor

This inexpensive component drops voltage to the auxiliary cooling fan to provide a low speed and has been a weak point on BMW's for years.  Why they haven't come up with a better solution is beyond me, but it seems to be a pretty typical BMW response to a design problem that doesn't appear till after the warranty is up.
The same issue applies to the HVAC fan speed control, although the latter E34's now have the infamous "sword" or "final stage element," which appears to be a either a transistor-switched resistor block or a PWM (pulse width modulated) motor controller.  This improved technology apparently does nothing for the reliability but does increase replacement cost substantially.
It seems to be important that you fix the aux fan resistor as soon as you discover it has failed.  I (and another owner) have experienced failure of the aux fan fuse which may be a result of the resistor failure.  During A/C operation (or in the event of engine overheating) both the low and high speeds are used in sequence.  It is possible the transition to high speed places an excessive load on the fuse without the low speed being operational.
I had the dealer replace my resistor as my car is still under the 4/50k warranty.   They removed the front bumper to allow access and did a flawless repair job.