No heat from passenger side

May have a stuck blend door or leaking vacuum line/solenoid or electrical connector. Also, check the switch/controls on the passenger side. If the problem isn't in the switch, you'll have to remove the HVAC unit and inspect the blend door and it's controls.

If you aren't already familiar with this procedure, I don't recommend you do this yourself. However, if you do want to do it, I recommend purchasing a repair manual for your car. It'll have ilustrations that you can refer to as well as helpful hints.

Here's the repair procedure. (It's pretty involved)

Unfasten water hoses (1) to end panel. Carefully blow out lines to clear remaining coolant from heater.

Draw off coolant and unfasten both tubes (2) on the end wall. Unfasten screws (3) on the end wall.

Remove steering column.

Remove trim from instrument panel.

Remove center console.

Remove reinforcing tubes from left and right sides.

Remove heater.

Installation:
Do everything in reverse. Hopefully you kept track of all the bolts.

Robert Bentley publications has an excellent manual for the E-38 body style cars. I agree that the blend or heater door is not opening and it may be an actuator, the proprietary scan tool from BMW has the ability to go to the level of seeing whether or not all the doors are opening and perhaps it has logged a fault that the proprietary scan tool could bring up and point you in the correct direction and allow the most efficient use of the repair time.

As soon as I hit the send button, I realized the issue might also be the heater control valve on the firewall, feel the output tubes during operation to see if they are allowing hot water to flow through.

The heater control valve is on the firewall, just left of center behind the engine top cover, there is one in, and two out tubes. Feel them all at operating temperature and see if they are getting hot water flow.

Lets get past the "if" When you have the facts, let me know.