Yes, the int light comes and when the pass door is opened. Also, if the drivers door is open and the passenger door is opened, the little "puddle light" comes on in the driver's door.
I think this is the seat belt relay. My question to you or anyone else who knows about this is, can I just bypass the seat belt relay, and rewire the circuit so that the light comes on when I open the driver's door? I understand the little red flashing seatbelt lights would probably not work.

About my alarm issues. I have solved the problem. Upon removing the lock cylinder in the drivers door because the key was getting sticky going in and out, I noticed it had been replaced and had a little "R" engraved next to the Mercedes symbol. However, I recognized the drivers side is the left side in the US. Pulling out the pass side lock, it had an "L," so I swapped them and now the alarm works perfectly. Very simple to remove and change. I guess both cylinders were replaced, and there was some confusion as to being a UK spec car and US part #'s.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro