I just found that the passenger side rear window does not go back up. The motor runs. The window goes down but must be pulled up by hand. How do I remove the door panel to access the problem? 1990 300SEL

Rear Passenger Power Window Operation
The rear passenger power window switches receive battery voltage through the window lockout control circuit if the window lockout function is disabled. Ground for the rear passenger power window motors is supplied through normally closed up and down contacts of both the rear passenger power window switches and the driver power window switch. When the either rear passenger power window switch is placed in the down position, the power window motor down control circuit is switched to 12 volts and is applied to the down side of the rear passenger power window motor. Since the other side of the rear passenger power motor is connected to ground through the normally closed up contacts of both the rear passenger and driver power window switches, the window travels down. By placing the rear passenger power window switch in the up position, the polarity of the power window motor is reversed and the window travels up.
The rear passenger power windows can also be controlled from the driver power window switch.

Did you try pushing the lock out switch on the drivers master switch ?

if you hot wired it straight to the motor and it works up and down fine chances are you have a broken or shorted wire between the button switch and the motor itself possibly between where the door opens and shuts at the hinge

if its power window you may try this turn key on hold window button to up while at the same time with the side of your hand rap on the inside bottom and rear middle area of door panel if not your gonna have to pull door panel to inspect window motor

If you can raise and lower the window by hand, then it's the regulator. 9 times out of 10 it will be the regulator. I have a 2003 A4 and both rear window regulators are broken.

more than likely one of the switches are bad (down side of switch works but up side of switch dont) being as the switches are wired together if one goes bad other one won't work either take the switch out one of back doors should be same as front passenger door put rear switch in front passenger door if works then u know front door switch is bad if it doesn't then drivers door switch is bad
u replaced window regulator Did that come with motor too??? if not check to see if motor is good
there is two wires going to motor cut isolation back on wires and using some jumper wires attach one wire to positive terminal of a 12 volt battery attach other wire to negative terminal of battery window should move up or down reverse connections and window should move other way

The motor on the passenger side is shorted and is now tripping the circuit breaker. Odds are, the back window motor is also bad. You have to take the panel off to check the motor. You can power it directly to verify it has failed.

either the window seal is putting to much pressure on window, but most likely the window motors are wearing out.
This always happens when motors wear out. They will go down fairly easy, but don't like to go up because of the pressure of the seals on a weakening motor, they strive/struggle to move up

mine does that. i just pull the switch a little harder on drivers side and it works

it is much easier to replace the motors then to deal with having them off the tracks and chancing breaking the windows when you go over a bump you can usually find the window motors on ebay for under $20

The problem may be in the wiring which if you pull up the carpet on the front passenger side by removing the door floor trim and lifting the carpet you will see the wiring underneath. Probably laying in water or has been. Look for splices in the wiring which will have corroded. Repair the wiring then check the fuse.
Another problem may be in the motor found inside the door. The brushes may have worn out. If so you need to replace the motor. Ususally the drivers side will fail first since it is the most used.