Sounds like you did the right thing with the alternator/ammeter wire. If it's not too late I would make sure that crimp is real tight, and that you Tape-the-living-crap-out-of-it (since it is too late to shrink wrap it if you didn't) as a safety/longevity kind of thing.
It's probably fine, but I just like to make double-sure of the connection and make sure that there is nothing to abrade the wire's outer jacket that might cause a short.
I know Ford just used a push-pin type connector, and it seemed to work just fine for 30 years, but there's a lot of potential current passing through that wire, so caution is the best route to take. Of course if you used fusible links or fuses at each end of the wire, that is normally all the protection you need. And it's not always obvious that BOTH ends should have one.
2) I ended up using an O.E. type connector on the lighter wire so we could retain the stock lighter. Plenty of them in the old harness, and easy to find at your local auto parts store. Usually.
I had them already in my "kit" so it was no big deal.
1) Yes, use the "dome lamp" and ground wires from the windshield (wiper) harness for your "courtesy/map light". Since the stock courtesy lamps were attached to the steel dash, the ground is not specifically necessary, but the factory "dome" lamps were in the plastic wiper cover so it was needed. Just hook it to the dash with the light's screw, and it'll be another ground. Never a bad thing.
The dash light wire is for for the '73 and later dash knob indicator lights, so it's part of the dimmer activated instrument lighting circuit. You can use it for aftermarket gauge lighting as well, so don't cut it off. Just seal the end if you don't have a use for it right now.
4) OK, just went out (brrrrrr, he says in his best CA accent!) and got a new one out of my trunk along with a multi-meter. But it just happens to be marked. With the center terminal in the up (12 o'clock) position, it looks like the top or center one is for power. The one towards the driver's side is for LOW speed with the opposite one for HIGH speed. A quick continuity check bears this out.
5) Don't remember which color is which (assuming the stock wires are still on it), but I think the terminals are marked "+" and "-" so you can use those same wires from an aftermarket radio. You just have to either remove the speaker to look for the markings, or do your best contortionist impression to see if you can see them. They may even be different sizes, and if so, somebody who does a lot of installs may be able to help interpret that.
If a stero is used, you can actually use it as a "center" speaker by wiring it using jumpers from your regular stero speakers. Don't remember if you use both positives (think so) from the stero, or both negatives, but you hook the same polarity wires to the normally opposite polarity terminals. At least that's the way it was listed back in the '70's (if I remember correctly?). Not sure if that's still valid or not.
Really, a speaker that old will probably work no matter which way you hook it up. Just may not be as efficient if hooked up backwards. Dunno.
Or you can just use it for your CB radio's external speaker jack.
Hope you can make sense of the ramblings. Good luck.
Paul