Alright, here is what I know:

1) Yeah, I guess I do know the Explorer, I'm, err, kind of a car guy. I don't even own an Explorer ; )

2) First of all, there's no way you're going to get RadShack to make you anything. All the guys are paid on commission, and could care less what the hell you're making as long as you buy something. Keep that in mind when shopping there. Don't worry though, making the box should be pretty simple, especially if you're at all familiar with a soldering iron (or are a quick learner like you said). It should consist of this:
a) Wiring (from scratch or by cannibalizing RadShack's unit) a working switch system, without the container. Just put all the pieces together and try it out (you can test this on your home system if you've got anything with RCA on it, just connect the main out to your tuner, and a cd player or even a walkman with a 1/8" (stereo mini jack) -> RCA adapter wire (RadShack ~$2) to each set of inputs. Build it and tweak the thing until the switch works well.) Once you've got a working switch, you've got to make it "car-proof." Unless it sits on the seat, it's going to shake, and it's going to vibrate. I'd make sure every single soldered joint is done very securely, and taped with electrical tape. When you pick a box to put the switch in, make sure you don't just drop the guts in, I would secure them to the walls of the container, and then wrap your automagical switch box with closed-cell foam padding to keep it from bumping all over the place. (*Please* don't tell me you do any serious off-roading with your Explorer, please?).
b) Once you've got a working box, the installation of such should be easy. It would be simple to close the box and run wires to the switch, that way, you could put it anywhere you want. The location of the box depends on where you want the switch. Pick a switch first, one that looks sexy, is easy and foolproof to flip, and that isn't so garish that you hate it after two months. Then pick a place for the switch. I'd say that the tissue holder, in the glove compartment, or in a custom mount. You could cut a whole in the plastic next to the second lighter outlet in the passengar footwell, or in the dash just to the left of the stereo and climate controls. (Incidentally, do you have an XL, an XLT (if so, is it a 941A or 945A trim package), an Eddie Bauer, or a Limited (or is it a two-door)? That would help, because the consoles are different with the different packages: trip computer, etc.) This would be a one-shot solution (i.e. you're selling the switch and the box with the car, unless you keep it until it dies), and I would think that you would want the option to put it in another car if you don't keep the Explorer. Sooo, I would say that a teeny hole in the glove compartment would be good, or maybe in place of another dash control? You could pull the fog light switch and put it there, and run the fog lights to turn on whenever your lights were on - that might look very professional ;) If you mount the switch any of these places, I would say to put the switchbox guts in as slim or small a box that you can, and tuck it up above the passenger footwell, or behind the console (you should be able to wedge it in pretty well, though you might have to pull out plastic pieces to get it in, pretty easy though. There are more options though... You could put it in the tissue dispenser, or (ooh, this might be really sexy, and hidden!) in the bottom of the ashtray. I small switch would fit *really* well in there, you could access it intuitively while you're driving, and it is hidden to distract any would be thieves from thinking there's anything especially fancy in the car (though I don't think that's going to be the case with a nice, shiny empeg sitting in there!) Still the ashtray would be a great place as long as you don't smoke. Geez, you could even put a small rocker switch in the hole for nickels in the coin dispenser, what do you need nickels in there for anyway?? The switch could even be tucked in with the changer in the center bin. I would say that the ideal place for the box with any of these locations (besides the ones in the dash) would be under the passenger seat. Wires could be run under the carpet (wrap them thickly though, don't want them wearing bare) and to a switch in almost any location your heart desires. You can put almost *anything* under that rear bench seat, but it's a little far back to run a lot of wires to everything. I think your best bet, and most professional looking, would be to build a box, and then run wires to a small switch wherever you decide to put it. That way, you have lots of freedom with the actual "press me" switch location, and you can run the wires to the bulky box and put that just about anywhere. There, that should take care of the switch :)

3) Whoa, before you go taking out the whole head unit, think about what you're getting into. That is a custom JBL unit, in a non-standard opening. Putting an empeg there would be a project unto itself (you'd need an adapter from Crutchfield, at the very least, and redoing all the wiring would *suck*). I won't even *go into* finding another place in the car for the head unit. Think of it this way. You have a perfectly professional, *very* nice looking setup already, all you want to do is mount this new component somewhere, right? Don't make it tons harder than it is (though I know that's hard, you should see some of the stuff I've come up with to put the empeg in my teeny Honda! ;) Making and installing a switchbox is one thing, but completely re-engineering your entire car audio setup is something that (in your position) I would trust only a professional shop with. Approach the problem this way: You can keep all you've got (tape, AM/FM, CD changer) and add an empeg and a switchbox. This ain't an afternoon project, and could take a couple trips to RadShack and a weekend or two. Be patient, and don't cut corners. I know how much you want that thing in the car, hell, after a few hours, *I'd* be tempted to duct tape the infernal thing to the dashboard! Just think about it before you cut any holes, and lay it all out where you want it before you finally cut any wires (leave yourself *lots* of slack...it's *real* easy to tuck away extra wires, but *real* hard to stretch them ;)

4) Hmmm...installation of the empeg itself. This is a toughie. You want to be able to see Hugo's neat graphics (so do I!!), but the engineers at Ford didn't take that into consideration when building the Explorer (doh!) So you're left with very few options. I do have a solution that I think would take care of 1) easy to see, 2) very secure, and 3) out of the sun (good plan!) so here it is. Mount it on the ceiling. I know, it sounds ludicrous, but hear me out. It fulfills all the requirements, and unless you have a sunroof, should be *possible* to do. You know those really neat custom enclosures that are sold in catalogs to hold VCR's, TV's, etc. in conversion vans? I'm willing to be that with a little patience and clever cutting, you could get one of those that would fit the empeg, and look pretty cool hanging from the ceiling. The only problem is that overhead console. Hmmm... I would suggest JC Whitney or something like Saleen makes parts for the Explorer. Even a conversion van catalog may have something that would fit your needs. Try buying a Car and Driver/Road and Track/Motor Trend/Automobile magazine and looking at the ads in the back of the magazine. I'm sure you'd find more than one little ad that has aftermarket parts for Explorer or Vans, for a couple of bucks, that could be just what you need. Anything other than that is going to take some thought. I'll get back to you.

Good luck.

Festus