Another hint : after you remove all 4 bolts and the unit is loose, then remove the 2 aluminum screw things that the bottom bolts went in. This will give you the needed clearance to then remove the units from the car.

To re-install, get the units back in, then screw the aluminum nuts in all the way, snug down the upper bolts, then screw the aluminum nuts back out until they just touch the mounting tabs, reinstall lower bolts and tighten, then tighten the top ones.