You model year should have the sensor in the door catch where it latches onto to close the door.
the sensor is in here:
Look at the opening in the middle of the catch, there should be a button inside that you can press with a screwdriver.
Instead of holding hte button with a screwdriver you can try putting a little piece of duct tape in the opening to hold the button just to test if this is where the problem is. Drive around with the sensor/switch depressed and see if the message comes back.
If it doesn't then the fault is somewhere along the wiring to the ccm/cluster display. You must remove the B-pillar pillar panel that goes over the seatbelt and follow the wire behind there to check for any breaks. If it is broken, the CCM will think that the door is always open. it must be repaired to make the message go away.
I think in 1990 BMW changed the door open warning to cover all 4 doors instead of only the drivers door. So you will have to follow this procedure for all four door catches. eventually you will pinpoint the problem door catch or problem wiring and be able to service it yourself.
Last edited by paperplane94; 01-21-2012 at 05:32 PM.