Couple of things to look at.

Check the power at the door terminals as you are thinking. The wires that go thru the door at the hinges can break after a 20 year life. So if you get no power there then you should check the wires at the door hinge.

2nd the actual mechanism can warp and not work. It jambs the mechanism and prevents it from working. My daughters car had this in the rear windows. the cast alluminum regulators were warped.

3rd The slide at the bottom of the mechinism can go bad and jamb. Thsi can be replaced but you must remove the regulator to do it.

4th The motor can fail but usually does not happen and if the lights are dimming then they are probably working, or at least drawing electricity.

5th With a schematic and a Ohm meter test the consol switches to make sure they are working correctly.

6th check the continuity between the consol switch wire harness and the terminals on the door.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car