Hi Gus,
I've retro fitted 968 mirrors onto my 944. If you look on the bottom of the mirror casing, there is a hole. If you shine a torch in through the hole, you may be able to see a large wheel with slots cut into it. What you've got to do using a screwdriver is turn this wheel. I can't remember which way but it's about a 1/4 turn or there abouts. Once you've turned it, you need to slowly weedle the mirror out of the mirror mount. Be careful because replacement mirror glass is around £50. You should be able to get a couple of fingers in behind the mirror. When the mirror pops out, there will be a couple of spade connectors to take off the back of the mirror for the heating element. Once thats done, the motor is right in front of you. Just make sure non of the plastic motor mount has popped out. It's more than likely that this may have happened causing you to lose the up down movement. Failing that, mirror motors pop up on eBay regularly for around £15 - £20 new.
Hope this helps.
Ben
