fuse 21 on an e30
With the key off, pull the negative lead from your car battery and place an ammeter between the lead and the battery post.
Anything more than a very small current indicates a short somewhere.
Next, pull and replace the fuses one by one until you find the circuit causing the drain.
Glovebox light
Flashlight
Ignition Key/Seat Belt warning
Interior Lights
Radio Memory
Trunk Light
Active Check Control
Service Interval Indicator
OBC
Fuel deliveryTachometer/Fuel Economy Gauge
Digital Clock
Multi Function Clock
Clock and radio memory are always drawing power.
Later,