If the car was shut down, then the pant motor should run against the micro switch limit. That's all the way down. Then when the motor runs when the car is turned on, the pant motor runs against the other limit and the pant is all the way up. If in the off position, pant all the way down, snugging the screws doesn't result in the pant to go all the way up when turned on, then something inside is preventing the motor and lead screw from going all the way to the up limit. It's mechanical with two limits, so the motor should automatically run from limit to limit. There aren't any loose wire rods, are there? There are a couple of wire rods at the very bottom that plug into the bottom pivots that once inserted in their holes are bent over a little to prevent them falling out. I'm not sure what exactly would happen if one were to come loose. I think they tie both pivoting bottom rods together because only one of them is directly actuated by the motor and lead screw.
That's all I have short of watching it run on the inside roof to see if it does go through the action.