Hey Schmidty...
Glad the info worked for ya!
If you do not already have a sub, it is not wired for one. It would go where the little netted storage compartment is on the pasenger side of the rear of the truck.
The grille on the tailgate is a vent- there is a rubber vent behind it, so that when you close the doors or tailgate, there is somewhere for the air to exit the vehicle.
The good news- if you have access to a junkyard or you feel like eBaying, you can get the factory sub box, and replace the stock subwoofer (I put a JL Audio 8" sub in mine) and wire it just like a sub with an aftermarket amp, and not lose any cargo space. Several members have used that setup, and I'm actually going to do a writeup as I have to reinstall it into my new Explorer shortly. On your '97, the radio amp is in the rear by that storage compartment. If you use the 1998+ 8" subwoofer box (they were 6" before that) you will have to use the mounting space on it for your stock amp since you're using the wires running through it for your radio. You will need to mount your sub amp under the rear seat or where you'd like it, and just run the speaker wire to the sub in the enclosure. When you get the sub enclosure, you can also get the stock grille that covered it, and it looks completely factory but it sounds much more awesome.
Don't underestimate the 8" with a polyfilled enclosure- unless you really want to turn heads, it's an awesome setup that leaves you all of your cargo space.
This is what the factory setup looks like with the subwoofer grille vs. your cubby hole:
For your high mount stoplight, you have to drill the rivets holding the black trim that surrounds it (open the window and look up) and remove the acorn nuts. Then you can get to the light. The ballast may be bad, that you can get to by removing the rear hatch interior trim and it's the silver thing right there behind the light.
Here is a thread with some pics...
http://www.explorerforum.com/forums/showthread.php?t=237636