Primary causes of failure:
The first place to look are the motors themselves. The factory motors and gear units were crap. Plastic gears and rusty hinge components caused the gears to strip early on in these systems. Ice, mud and other debris would cause the hidden doors to jam. The factory 10 amp breaker for the electrical was too strong for the weak motors and gears and usually never tripped before the gears themselves stripped or the motors burned out. Poor sealing of the motor units allowed moisture to enter the units and caused premature damage.
Do you know if the motor units are the original ones? Have you loosened or disconnected the actuator arm from the motor unit in order to manually move the door? If not, the gears may be stripped which means you need a replacement motor unit.
It has been a while since I did mine so I'm not familiar with access to everything off the top of my head, but I would start by testing the motor. Get a test light and disconnect the plug going to the motor. Ground your test light and then use the probe end to touch the prongs in the plug end coming from the main harness. Have someone turn on the lights (with the key ON) and see if you get power to the test light. Since you said one side is working I'm betting you get power to the dead side. If not, it gets more complicated tracing the break in the power source. It could be a stuck contact switch for the door or a bad relay or breaker. All of the headlight door wiring runs to a central mount containing 3 relays and a circuit breaker. Without seeing your car in person it is very difficult to diagnose and offer help online. It could be as simple as a connection.
While you have the plug for the motor unit disconnected, find a way to send power to it with a test wire or stand alone battery. At least you will know if the motor unit is working.
I was able to find a wiring diagram online when I rebuilt my door set up. I'm willing to bet someone around here has one or I can search my computer at home for it.