As I just did this and also talked to Gamiviti a bit about it I'll give my input:
If everything works but you don't have forward and backward then it's most likely the plastic gear.
WARNING! This will break the gears even if they are not already!
If you're prepared to fix this anyway, lift the front of the seat all the way up and take the two screws out of the little decorative front panel and take that off. You will see 2 nuts, one on each side. These are on the end of the threaded rods that run through the motor. If your gears are stripped then you can turn these (like a 1/8 to a 1/4 turn a side back and forth) and it will move the seat forward and backward manually. HOWEVER! If you do this, THIS WILL ABSOLUTELY STRIP OUT THE GEARS.
I have seen several videos online where people demonstrate this to manually move the seat when it doesn't move, and it will, but this will strip out whatever is remaining of your stripped gears completely. But it does let you at least move the seat into a position and leave it there if you end up having a bad motor or switch that you're unwilling or unable to fix.
That said, if you do this, then it will free up the little worm gear end (which is metal) and then you should be able to push the button and hear it engage. If you hear the solenoid engage then I'd just proceed with the gear replacement as you have a high change of a 100% fix with new gears. If you don't hear it engage AT ALL then you either have a switch or motor problem which is above my pay grade at the moment.
I had mentioned that the disassembly/reassembly video was a little misleading in that if you perform the steps I mentioned even by accident (for instance if you just quickly install the gears with a small impact like I did the first time) you will ruin the NEW gears. The design of the mechanism seems VERY engineered to make those plastic gears be the absolute point of failure and be a relatively inexpensive replacement part.
That said, when assembling the bottom of the seat I would suggest removing the small plastic piece that holds in the threaded rails on the bottom of the seat (2 phillips screws) and removing the entire assembly. Once you're done repairing the gears, reinstall everything in this order:
1) Install washer on threaded shaft, install motor (or secondary gear mechanism) onto shaft while they are completely separate from each other, install washer and lock nut, tighten nut.
2) Install the interconnecting shaft between the motor and the secondary gear mechanism.
3) Spin the rail connector plates to the same position.
4) Put the entire, complete assembly into position and replace the rear plastic connectors on the rails.
5) Replace the 4 front mounting screws.
