'84 300D Door Lock problem

I'm having a bit of an odd problem with my car. The front passenger door lock is acting up. It will unlock just fine, every time. And all the other locks in the car work fine. However, when I open that door and close it before I try to lock the car, it works fairly consistently, I'd say about 70% of the time.

I've read a few of the other threads on vacuum systems and door locks, but I thought I'd ask anyway since I didn't see this specific problem addressed directly.

Also, one other unrelated problem - my odometer panel lights up VERY dimly when I turn the headlights on. I have adjusted the rheostat (at least I think that's what it's called) - the little knob that adjusts the lighting - but it doesn't really help.

Any advice?