The 155 mirrors are a bit of a nightmare full stop! The internal casting sieze up easily. Best bet is to move them regularly.
1. remove the rubber boot covering the little lever for altering the angle, just pull it off
2. unscrew the silver ring
3. the black plastic cover pulls off, its very tight though. I THINK the 2 clips are about an inch from the top and the corner nearest the front
4. undo the 3 screws and the whole unit will come off.
You may still have to get behind the mirror after that to get to the ball and socket. Let me know if lubricating the lever doesnt work. I posted how to remove the mirror a while back, will do again if you need it.