The up and down is controlled by a limit switch. switch has a gear that rides on the drive gear.
You can remove housing without removing whole motor. There are four screws in the bottom of housing. The screws go in from ground- up.
Careful when you remove housing. the gears can easily jump out of housing. The wires are typically tight. and are routed under the gears to the motor itself.
I have found broken solder joints from vibration.
When you get into motor, check wiring to switches,ascertain that your ground is good. If no movement in either direction, you have electrical problem, typically ground. If ground is good, you will be able to go in one direction or the other.
You can always run wire to battery via your wiring harness to tow vehicle.
as long as you dont touch grey wire, which is ground, you can check motor. The switch is a simple three position double throw switch. Cost about twenty dollars at Grainger.
Insure that motor is bad prior to removing motor, it is challenge to get motor back into housing with taught wiring. And if you really get inquisitive, and decide to take motor apart, watch for the brush springs taking off on you when you seperate shaft from field. (experience speaking here).
I have found that rarely a DC motor has gone bad. it is usually in the wiring and more often than not it is a ground problem.
The limit switch is a plastic arm that closes microswitch when extreme rotation is attained in either direction.
let me know if you need help in resetting limit switch.