Alright Don, I finally got my door locks working last weekend after a year of work...
The first thing I would do is to take out the rear seat and make sure that the vacuum pump is getting around 12 volts. Here's how this can be done (I'm assuming that the system is the same as on my 1988 300e and 1988 420sel):
When you take the seat out you will see the plugs attached to the pump. One will be a yellow tube (for the vacuum). One will be a rectangular connector with 2 wires attached. This is the power source for the motors. The third connector is round and it has three wires attached. One will be green, one blue and one yellow. These three are what tell the pump to turn on. The green wire comes from the passenger door; the blue wire comes from the driver door and the yellow wire comes from the trunk.
These wires come from activators in the doors and the trunk. The activators each are connected to 12 volts, and when the door is locked/unlocked, the activator sends out a brief +/- 12 volt pulse to the vacuum pump. This tells the pump to turn on. The actual power comes out of the rectangular plug.
Your problem is probably not due to a vacuum leak. The pump turns off when the proper pressure is reached so if you had a vacuum leak the symptom would be that it would not turn off.
First off, take the rectangular connector off and measure the voltage between the brown wire (ground) and the red wire (12V). It should be around 12 volts.
Next, remove the round plug and measure the voltage between a lock wire and ground. It should also be around 12 V. If both measure 12 volts, then you can assume it's a pump problem. You are lucky because you have another pump to test my theory.
Now if either of these are not 12 volts the problem gets a little more complicated...
If you remove a door panel, you can get access to the lock actuator (black plastic thing with a round plug and a vacuum line very similar to the one on the pump.) This round plug has three wires. A red wire (12 V) a brown (ground) and another that goes to the pump. Disconnect this plug and check the voltage between the red and brown wires. If they are 12V, then your actuators are all bad (which is unlikely). More likely is that the actuators are not getting 12 V, which means you can tear your car apart some more.
The power for the door lock system should come from the drivers side footwell. There's a box there when all of the optional equipment is powered.
Anyway, if you've gotten this far you probably know what to do...
Check the voltages and then you may have to splice back into some existing wires... Or run new wires like I did. It was kinda a nightmare, but now I guess I now all about the door locks.
Oh, for your wife's car - I'll bet it's just a bad door actuator or something got disconnected. The actuators are cheap (maybe $15-$25). I've got an extra for my 300e that you can have if it will fit.
Anyway, I hope this helps. If you need more assistance I'll be happy to help the best I can.
Best of luck,