About 95% of these type of problems is a bad ground. My experiences show that most people only focus on the source power not the return. Frame to engine grounds usually increase efficenity in Automotive systems. I'm OLD MOPAR/Chevy guy new to FORD! Still working on my 1988 E250!
Had to share to say thanks for the help I have received.

 

Have checked the grounds. Even ran new ones and it didn't matter.

 

One second thought... there is a faw inside the motor and or motor housing, perhaps corrostion. I'm new to FORD but sometimes a thin copper strap is used from motor to frame. This is a generic observation. I hope this helps.
BilldarrAZ

 

If you unplugged (the mounted non working reg motor) and plugged in an old unit and it now works, would 'seem' that the the newer unit is defective.

But,,,

Since the old unit was once removed for the same failure/condition would be suspect. If the 'hanging' regulator works, and you again plug the connector back onto the installed regulator and it still doesn't work would again point towards a defect.

If 'both' new/used units appear to be intermittent, could be a harness/connector open issue. If its really an intermittent, you could always add a piggy back harness off of the regulators connector and out of the master switch area. On a fail condition, add/remove power/ground to the pig tails and see if the unit works as it should. If it does, it would point to the master switch and/or door jamb opens as part of the diagnosis... If you've added power/ground 'direct' to the mounted reg as part of the testing and it won't move, then there's no doubt on the cause...

 

Well I put in a new motor this morning and it works. I had tried direct power to the switch. bypassed the switch and to the motor and it wouldn't work. I ran new grounds and it wouldn't work. The old motor I took out, will turn when hooked directly to the battery but groans in one direction. I guess it's just a waiting game now.

 

There are times when in spite of our best efforts "Murphy" gets in there !
Are the motors the same for both the driver's door and the passenger door?
This is a last desperate move and may help solve the saga of the fleeting motor.
I know this can be a bit of work, but if you have no problem with the passenger window I would consider putting that motor in the driver's door and see what you get. I don't know where you are getting the replacement motors, but it's time to suspect them and this is one way to prove it and stop looking to put a zipper in the driver's door panel. What do you think about this?

 

The motors are exact opposites.

 

So much for that idea.
Me thinks your best next move is as Hayapower suggested. It will take a little doing, but by this time you are an expert removing and installing those motors and don't need any more practice so his suggestion coves all the possibles and you will only have to do it once , unless you think you need more practice,,,,,,,,,,,,, just kidding !

 

Just hoping it keeps working this time.

 

Don't want to jinx myself, but it's still working after three weeks.