Okay, finally got around to this. Car is an 88 Formula, originally equipped with manual mirrors (and windows, and locks, if it matters).
First off, the original passenger-side housing:

The passenger side power housing from a fastback, don't remember the year:

The mirror bracket is installed.
You can put the power mirror bracket into the manual mirror housing, like so:

You can also see the round hole that the wire passes through is present on both the manual and power mirror housings. On the manual housing, it's covered by a piece of tape or paper or something, I easily poked through it.
The driver's side is a different story. My (extremely gross) manual driver's mirror, with the cable removed:

Notice the oval hole for the cable passthrough. If you're ever looking at a housing and want to know if it's power or manual, look at that. Power (or power-capable) housings will have a circular hole, manual mirrors will be oval.
Power driver's mirror from the donor:

You can see how the mounting points for the glass are different as well. I didn't tear this one apart as much, so you can see the grommet and the wiring that's attached to the motor, which is less than a foot long.
Just for funsies, here's a picture of the motor, and the harness extender that's hidden inside the door.


The connectors at either end of the extender mate to each other, that's why I call it an 'extender'. You can plug the mirror motor (removed from the door) into the cabin harness just fine, but the harness won't reach once it's installed on the door, hence the extender.
With the extender, motor, the bracket seen above, and the glass, you could convert a passenger side manual mirror door to power. I don't know if the glass itself differs, but I believe it does. I haven't gotten my hands on either power or manual mirror glass for the passenger side yet, but if I find some I'll update this thread again.
[This message has been edited by boxen (edited 05-30-2015).]