the locks on your doors are held in by a little clip and that is all. you need to buy the solex lock for your make and model car so it slots in properly to the hole in the door (i.e. it slots in and turns) then there is a little clip like the one in the image below that holds the lock to the door.
if the solex lock can be popped out, chances are the original lock would have been popped out in the same manner. solex do make locks specific for the u12, which are a straight plug and play fit, so if you're looking at other forums for cars that are more frequently nicked, then chances are the standard locks would also pop out for that make and model of car.
but again. if someone wants to break into your car, 9 times out of 10 in a u12 they will break the rear quarter window in your back door.