In your original post, you stated that the power windows, power locks, and power mirrors did not work, even when you replaced the switch (which one?). Your subsequent posts all refer to the power mirrors. Did you get the power windows and door locks fixed? What was the problem with them? And forget about looking for the 20A fuse -- there isn't one. It's a mistake on that diagram. The power mirrors get their power from the 7.5A fuse. Check the fuses while they are in the fuse block. Use your voltmeter to check them by grounding the ground probe and probing the little metal contacts on the fuses with the other probe. If you're getting battery voltage from each of these contacts, the fuse is good. I'm pretty sure your problem is not the fuse. It is probably a broken wire between the BCM and the mirror switch. Did you check the wiring in the bundle going into the door as Andy suggested?
If you did indeed get the power windows working, splicing into the feed for the windows is an option. The windows are protected by a 30A fuse, so the mirrors will be a minor load on that circuit.

 

My apologies, chevywaldo. I just noted you jumped into a post from 15 months ago, as you have a similar issue, but not exactly the same as the OP issue.
The remainder of my comments stand, however. If you have checked the fuse and found the fuse is good and is feeding the power mirror circuit, but the power is not getting to the mirror switch, you most likely have a break in the wire between the BCM and the switch.

 

I checked the "BCM ELEC" 7.5A fuse in the BCM and it's good. Not only is it good - when I pull it out - it kills the instrument cluster lamps and the ignition switch lock, etc. So it's good for sure.

So it appears I have a break in the wire (seems impossible) between the BCM and the power mirror switch (orange wire).

So it seems to me the easy way to fix it is to remove the drivers side door panel to expose the factory wiring bundles, then splice the 12V Hot wire from the Power Windows switches to the orange wire that feeds the power mirrors switch. Not a big deal. No one will ever know - will be hidden behind door panel I don't see any problem with circuit protection or fusing either. Power mirrors don't use any much power and they have their own fuse anyways.