My son has an 08 Dakota and the other day he put the DS window down and it stayed down. PS works fine. I pulled off the door panel and looked for broken wires, none. I checked the switch with my VOM, it seems ok. Motor seems OK. I couldn't find any reason for it to not work. I took it to a shop, they can't find any reason for it to not work. What are we missing?

measure with a ohm meter to if there is resistance on the motor- if open- there is a PTC ( electronic fuse) inside that has gone open.

A permanent magnet reversible motor moves the window regulator through an integral gearbox mechanism. A positive and negative battery connection to the two motor terminals will cause the motor to rotate in one direction. Reversing the current through these same two connections will cause the motor to rotate in the opposite direction.
Easy way to test that the circuit is capable of running a window motor is disconnect the plug at the window motor and use a head light or a fog light as a test light and connect those two wires to the plug that goes to the window motor harness connector. If the light lights up in the up and down position the wiring is ok and you have a bad motor. If not you have a power or ground issue.