Take a look here:
http://www.supermotors.net/clubs/fullsizebronco/registry/media/560656_1
note where it is saying these plugs are in the drivers kick panel? Don't worry about the fact this is '96 and has the signals. I suspect it's about the same or is the same plug, you just may not have the turn signal indicator wirings, but the plugs for the wiring for the mirror/motors should all be there. so look for plugs that look like that after pulling the kick panel (may have to also pull the aluminum ebrake plate, but that's easy). Next those plugs and then look at the color of the wires going in, they should match steve's diagram, maybe you'd be missing position 6 on each side which is for the turn signal indicator.
Edit: Wiring is an interesting deal with mass produced vehicles. I don't know about other makes, but I'd say Ford figured out that the path of least cost was to produce our Fseries trucks all with the same basic wiring harness. So we have wiring sitting there that may never be used since it was meant for a truck that had more options. In the end though, trying to produce 20+ different harnesses for the same F series was just way more cost than having it all sitting there, same thing, for each vehicle. THen plug in the options based on trim level.
dble edit: Probably also why ford would prefer you buy a truck with the most options possible. they get little margins on all the junk, and hey, the wiring was already there anyway.
Here's a bit more for you...my take is I see 5 wires total, but then there will be a split from your drivers side door control panel. there will be a left/right that will initially direct power to either of the 2 plugs depending on selection of L or R. the actual control beyond that is the same....left/right mirror motor movement, up/down movement and optional turn signal. So I can easily see the harness from the control panel down to the plugs (should be drivers kick). From there wiring over to passenger side. On second thought though, you might have to pull the drivers door panel...would be strange to go to the kick panel plug and then back into the door (mirrors mounted on doors so drivers side mirror should have wiring right up to mirror, no kick panel plug.