The RH front door assist stopped working. All others including trunk still works. Any advice on trouble shooting to determine where the problem is?

Try resetting vacuum pump which is located in trunk. This is only a temporary remedy.I believe it is the #9 fuse on my 1992 500sel.Remove and re-install this fuse and see if it works.Your vacuum pump or a leak in a line is probable culprit. Their is a lot of info here on door assist problems. Good luck.P.S. I reset mine 3 months ago,everything is still fine. Also check hose connection at rear trunk lock for a leak.

That did it, now let's see how long it will last!

The closing sssist pump is a common item to fail, removing the two fuses resets it but unless you are really lucky it will be back. The pump is located in the ares between the fuel tank and the wheel housing on the drivers side. i is tricky to get to but acessible by removing the liners. The bad news is the cost about $600. there are some rebuilts but...
The 140 had some issues the bose Amp, vibration ,and the AC all costly items, but great car.