There are two things that could be wrong with it, the up-down motor or the switch. Whichever way you decide to do it the seat will have to come out to give access to the motors underneath.
I would do it by removing the seat, sticking it upside down on the bench, and connecting 12 volts, suitably fused from a battery to the connector block. Then by use of a multimeter find out where supply goes and stops. It may just be a case of a wire come adrift.
Good luck