Interface - you got it.

Powering - looks good, two things: 1) Remember to put the fuse as close to the battery as you possibly can. The amp has a fuse to protect itself already. This fuse in the kit is designed to protect THE CAR. Any length of wire hanging right off the battery is a potential for shorting the entire electrical system (I killed a Mercury Tracer this way - fuse was under the hood, but too far from the battery ) 2) You might need some more connectors, or - a soldering iron, solder, and shrink wrap are even better. Overall the kit looks fine though.

Fitting - try some digging. You might be able to remove the glove box (it usually unhinges and pulls off) to get behind it. If you can't do that, don't use foam to support it, as it will squeak later!

Wiring - you are correct on that one, but don't forget that you also have to run the power wire from the battery all the way down to the amp as well... Sounds like your'e putting it under the seat. You will likely find some good bolts down there, a seat mounting bolt should work well. Make sure you get a good connection! Also, at all costs, keep the power wires away from the RCA cable. If they run along side each other, you will hear the alternator whine induct (?) over from the power wire into the RCA's, onto the amp, and through the speakers. More flex loom tube will help, or it is acceptable to apply duct tape over the wires to stick them to the car and keep them away from each other (acceptable as long as no one ever sees it that is).

Misc - looks like you got it pretty covered. tanstaafl put it well. www.crutchfield.com should have an online guide to tell you what speaker sizes you have. If you can, I'd replace them - you will get a HUGE improvement in sound with almost any aftermarket speaker in the $50 plus range in the 4" or $80 + range if it's 5-1/4.

Good luck! I think you'll be fine.