[loki_the_glt]

What hapens if you swap the wiring plugs over for the front windows?

If you swap over the front units and the passenger window works from the drivers' window switch then the wiring or motor's at fault; if the passenger window doesn't operate then the switch is the problem. Confirm by pressing the passenger switch: if the driver's window operates then the driver's switch is the culprit, otherwise the wiring or motor is defective.

Switch cleaner, available from Maplins inter alia, might help. Otherwise the fix depends on the source of the problem but switch problems can often be resolved by swapping them over.