check power and Gnd to the amp. Under the right front seat. there is an inline connector about 8-10 inches from the amp.
moisture can damage the pins.

Depends on the condition and how it was maintained. In BC, have the car carefully inspected for rust damage. Volvo cars have a great body rust treatment system. the better EU cars have similar, but the Volvo treatment is near to the best. A good technician can inspect the car BEFORE you buy and give...

What little sludge this does break up, will likely plug the oil pump pickup screen.
Pull the pan to clean properly.
when you see the amount of crud in the pan and how solid it is, you will understand.

R/R of the pan is not that bad, it is the remove of the crud that takes time.