This sounds like an electrical issue. If it where in a bind, then it would make the same noise / attempt to move in both directions, not just out.
The motor is not getting juice for some reason for the in direction. If it where me, I'd find the wiring diagrams, or at worst, trace out the system to see what parts are involved. Is it simply a 12Volt DC motor with power polarity being reversed at the control switch to change rotation direction? I suspect it is. And, if so, you have to know the right way to check the switch.