I think your problem is actually a wire problem or a ground problem.  I had a
similar problem with a customers van.  Turned out that some of his wires that
go from the switches on the driver's door out to the fuse panel were
partially cut and broken where the driver's door shuts.  Apparently the door
had shut on them somehow pinching and partially cutting the wires.  The
result, you could get 12 volts at the motor but it wouldn't work.  When I
tested for amperage there was very little.  As you can tell this was a rather
frustrating troubleshooting session until I finally cut open the rubber boot
on the wires that come out of the driver's side door and voila!  the
partially cut wires actually snapped completely so I had found my culprit.
Also I sell brand new power window regulators for $150 each.

This is the motor and the regulator that is made by the OEM of the power
windows.  These are updated and don't use the pulley and cable method like
the trouble prone motors, but use a screw jack method like the manual
regulators (trouble free).
The motor location is moved to behind the speaker in the door instead of
being in the bottom of the door.  This means if you have a stock speaker you
can get this to work.  If you have a deeper aftermarket speaker then you will
have some clearance problems (the speaker won't fit).  You can also get
shallower aftermarket speakers as well in which case it is not an issue at
all.

I also have OEM power window switches for $35 each.  You can get the frames
that hold the switches from you local dealer for a little over $3 each.

Hope this helps ya.
Ken Wilford
Van-Again
John 3:16