I just bought one of those slant-front dual-subs-and-tweeters behind-the-seat truck enclosures for $25.

-It's two channel (sub+tweet x2)
-It will be powered in a 4 channel setup (as rear two channels)
-It will be powered by a Kicker kx800.4 (100w x4 @4ohm, 200w x4 @2ohm)
-It has two good working top-mounted tweeters (specs unknown)
-needs two shallow-mount 12" subs. (came with two cooked garbage Pyramids)
-The two sides are separated by a divider.

Question 1:Will it cause damage to my replacement subs to feed them full frequency? I know I have used car subs to replace woofers in 3-way home speakers in the past with no quality/reliability issues, so I could basically use ordinary car subs in these right?

I am also wondering how much power those tweeters will handle. There is no branding on them or the box, so I don't know what they're rated at.

Question 2:How many watts would the tweeters be receiving if ran with a pair of two-ohm subs? With a pair of 4 ohm subs? What would the impedance of tweeters be?

Question 3: How can I have the subs powered at 2 ohms? Do I need dual voice coil subs? How would I hook them up?

EDIT: Question 4:
If I run this amp bridged (400w RMS x2) and just have the two full-range channels behind my seat, will I blow these tweeters to smithereens? Or will the tweeters' higher impedance save their little asses?

 

Given that they had pyramid subs, you have to assume the tweeters are less than stellar. Full range isn't a problem for the subs, but i'd use a coil to restrict frequencies to below 100hz or so, likewise a cap or simple crossover can be used for the tweeters to protect them...which they should have, unless they're piezo tweets which have a natural "xover". However, they are probably good for 25 watts before they blow. In order to have a 2 ohm load you need either a single 2ohm coil(not too common), dual 4 ohm coils in parallel(common), or dual 1 ohm coils in series(not too common).

 

They are cone tweeters (in a horn assembly). I will put a crossover on them anyway though. I wouldn't want to restrict the frequencies to the sub since it is a full-range system, and with no mid-range speakers, I would be missing the entire middle range of sound.

Suppose they can handle 25 watts, that would be 25 watts at whatever impedance they are. tweeters are usually higher impedance such as 16-32ohm, aren't they?

If I have a 4ohm sub and a 16 ohm tweeter parallel on each channel, then the sub would be receiving the majority of the amp's power, correct? Rather than 50/50 between the sub and the tweeter.

 

There should be a crossover in the box already. Probably just a capacitor, or if it is a piezo tweeter then you dont need one. *cone tweeter in a horn assembly* = no sequitur..

Correct power handling is independant of impedance.

Correct. but with such little information we cannot give any precise answers.

Why are you bothering with the tweeters anyway? Concentrate on putting good coax speakers up front and just run a sub in the back. Use 2 channels up front for L and R and then use 2 channels bridged in the back.
A proper subwofer wont cover much more than a few hundred hertz anyway so you will have a huge empty zone in the midrange untill the tweeter kicks in. Just like a home stereo focus on good sound upfront.