As suggested, get a multimeter onto the solenoids so you know if it's getting juice. I've removed the wiring to the solenoids and earth cable before, lightly sanded, rehooked it all back up and hey-presto it fired into life.
It could be a number of things, dodgy solenoid, electrical cable corroded (not unusual for the wiring to turn green with oxidisation) as manufacturers didn't used tinned cable.
Finally, the carbon brushes in the power trim unit could be worn out.