I used duct tape to hold the mirror in place since I didn't have an assistant. The duct tape came in handy to hold the nut in the wrench while I was getting it started on the threads.
Next time I will beg, borrow or steal a ratcheting wrench because of the tight fit, this will make life alot easier threading the nut on.
If your mirror got knocked off, you may need to chase the threads in the mirror's mount and use longer studs, my case I used a longer bolt as that was available at the auto parts store along with a washer.
A telescoping magnetic pick up tool came in handy as I was dropping nuts and bolts into the door, some fell out the bottom onto the ground. Can you tell that I've done this before?

Good luck, it's not difficult just tedious.
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jetman
Silver 86 SE 2M6 4-speed, with "check wallet light"