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I haven't bumped into your particular problem, but staring at the wiring diagram and with what little experience I've had, I may be able to give you some insight.
First off, the locking mechanisms are normally fairly stiff to move manually as all the motors drive a geared rack, especially the tailgate/trunk one as it has to move a slide pin, not just a flimsy linkage and plastic button. The locking mechanisms can get stiff if dirt gets in and makes gumbo with the grease to the point the wimpy little motors can't move the rack easily -not common in doors in my experience and certainly this would happen gradually. However, the slide pin in the trunk/tailgate can get stiff if it isn't kept clean and lubricated which would make that motor work harder than the rest.
None of the locking motors ground directly to the chassis except through the relay. If they're anything like the 240's, the locking mechanism motors should have a protective thermistor, a little smaller than a quarter, which acts like a fuse when the motor is overloaded for an extended period of time (like maybe more than a minute). This can occur when the driver door button is held down too long (like resting your arm on it for more than a minute or so while driving). Another possibility would be the relay sticking closed (although I have not heard of that). The thermistors do not reset, but may be replaceable if they are accessible (something tells me the 240's are externally accessible, but the 700's are internal). In this case you would hear the relay clicking and see power getting to the doors, but nothing would happen. Does that sound familiar?
So, in your case, what I think might have happened is that someone has a tendency to keep their elbow on the driver door button or that button has a tendency to bind in the door panel. The first motor to go would likely be the tailgate -as noted above because of the extra load of the slide pin. The other motors apparently went at some other time. Normally they would go one by one, but if the button was kept down long enough they would all go in one session. So that's my theory.
Now what you need to do is more electrical testing. Pick one motor where you can get at a door connector. You know how to do the front passenger door so pick that one.
Open the connector. You need to identify the wires going to the motor. They should be blue-red and red-white wires. Use an ohmmeter across those pins (the ones that lead directly to the motor). If you get anything less than say 100K ohms then you know that motor is likely okay.
As an additional test, probe the end of the connector block that goes to the relay. Put one probe on one of the matching motor pins, the other probe on the other (it doesn't matter which way). You should be reading zero volts (or close to it). Have someone push the driver doorknob down for a few moments then pull it up for a moments. In one mode you should see +12 volts (or any significant + voltage), in the other mode you should see -12 volts (or any significant - voltage). If you do then the relay is likely okay.
If there is any doubt, and because the relay is so easy to get at, you might want to first try replacing it with a used one (shouldn't be more than about $10). If you decide the motors are gone and that replacing thermistors isn't straightforward then a used replacement should be quite satisfactory -verify as not shorted and check for stiffness. You might want to clean and re-lube them with a thin coat of light silicone grease. If there is any doubt then replace one and test it before buying and installing the remaining three. BTW the thermistors won't likely be available at your neighbourhood electronic supply house so Volvo may be your only source unless you can identify the OEM.
--
Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day
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