Hmmmm! What a strange system the beloved General has chosen to make windows go up and down or more to the point NOT go up and down.
Bought my much loved 87 VL Calais not long ago and found only the drivers side front window worked.
Like many b4 me I assumed that with age the electric motors were worn out.
Turns out the problems were all Switch related. Wasn't going to worry too much about this as mine is a bit rough but goes like a cut cat until my Daughter's (pretty VL calais) suffered a LHR failure with the window 3/4 down.
Here is what I learnt fixing it.
If the rear window sw fails the main sw probably won't work either.
To isolate what's going on disconnect the pink,blue and yellow wires from the rear sw. (the sw just clips in to the door panel)
Nothing is live while the ignition is off. First up test the pink wire with a test lamp to check it has power (ignition must be on to test this). Make sure the isolator sw on the dash (in front of your right knee) is in the on position If you have power to the pink wire connect it to either the blue or yellow wire. Go shout yourself a packet of male spade connectors and some electrical wire to do this without the risk of shorting things out. Then connect the remaining wire (blue or yellow) to an earth point on the body. (the rear door latch is a good spot).
Turn the ignition on and you'll either find window movement or hear a click as the window tries to move.
I used this method to at least close the faulty window on my Daughters car.
It'll prove whether the motor is burnt out or more commonly the switch is faulty.
There is a way to bypass a faulty rear sw also.
Using proper spade connectors and about 5cm of electrical wire for each bypass connect the blue and green wires and the yellow and purple wires. Unless the front sw is also faulty this will allow you to operate the rear window from the master sw in front.
One more hint. If the windows work but are really slow try squirting some armoral into the window track. The slowness is most likely because the window track rubbers have gone hard and that alone will burn out the window motor. This worked well on my car.
I've attached a rough diagram of the wiring for the windows.