How do I open my passenger door on my Mercedes 300D?

If you are saying that the passenger door won't unlock with the vacuum system, you can simply pull up on the knob manually then open the door. The door locks all can be worked manually, as a backup for when vacuum fails.

If instead you are saying that the door is UNlocked and yet it still won't open the latch mechanism may simply be stuck. This is an unusual problem, one I haven't heard before. Even W123's that have been in bad head-on accidents that I find at the scrap yard have all their doors still working. If indeed it's unlocked and stuck, then one thing you can try is to simply put more force on opening the door from the outside. The outer door handles are designed to be pulled with a lot of force - the idea is that, in the chance that a bad accident causes a door to deform and get stuck, a rescue crew can strap the door handle to a winch and pull the door open. That's why they are "pull out" style instead of "pull up" style like most door handles you see on cars - ease of opening during emergency situations. While I'm not suggesting you do this with a winch, you can try tying a strap to the handle and putting all your weight body weight in to it. It won't hurt anything.

If all else fails you can remove the seat using the seat removal DIY guide to make access a bit easier.

Good luck and don't hesitate to be in touch if you need more help. My email address is in my profile.