When I insert the key into the trunk lock, is it supposed to turn full circle?
It unlocks at the usual spot if I carefully stop but it can be turned all the way around. It stays locked if I pass the magic spot.
On all of my Fords and Mercurys, the trunk lock releases at 1/4 turn clockwise of the key and stops at exactly 1/2 turn of the key. Something is wrong with your lock if it turns full circle.
Your best bet is to take the lock to a locksmith and have him replace it. He should be able to reset the new lock to your old key if it matches the ignition.
Mine still works fine but rotates in full circles. I like the lock smith idea lol. So far it's not a problem but it's going to **** when it is. I just got a popper mechanism so I can open from inside the car without removing the keys. Haven't installed yet but soon.