I'm not sure if i'm the only one or not but The other day at sonic i clipped my mirror on the menu and separated it at the joint on the door side. After dealing with it moving around at highway speeds for a few days i took it apart and tried to fix it. Let me tell you that it was definitely an adventure.
First- If you don't have any experience with these just go buy a new one or you'll wish you had by the time you get done.
After almost blowing my face out trying to get it off (The spring is under a LOT of pressure in there) I found that the three ball bearings that keep it in place had shifted. Iput them back in place and started to put it back together.
You will need the following tools from start to finish-
small flathead screwdriver
medium phillips (no. 2?)
pipe that matches the size of the lock ring to hold the spring under pressure
10mm socket
door panel removal tool(looks like a little shovel with a gap in the middle)
I used a ratchet strap but if you're good with a hammer you can try without it

Remove the door panel and the the 3 nuts that hold the mirror on and the plastic clip that saves you from dropping the mirror on the pavement (like i did...breaking the glass)
There's a circle piece of plastic in the bottom of the mirror under the joint about 2 1/2 inches wide, pop that out and there's your spring.
Wearing safety glasses, bend the teeth on the retainer ring out a little to release it. After you get it off use a pair of pliers to bend the teeth back down a little so they will grab when you reassemble it.
Pull it apart and reset the bearings in the holes on the base side then put it back together, using you're pipe (a d cell maglite is the perfect diameter) on top of the retainer ring.
I put the pipe on top the put a ratchet strap over the top and around the mirror joint area.
Ratchet it down till it touches the part in the middle it clips around and slap it directly in line of the pipe with a deadblow hammer and it should grab.
Put the mirror back on the truck and reattach your door panel. Provided you did everything right it will work good as stock.