No rear door experience here.

I can't see any reason why a set of super tweets wouldn't work just fine surface mounted on the dash and bouncing off the glass.

I mounted mine behind the original speaker grills in the body just rear of the front seats. If you want, you could put your crossover there too - just double-side tape it to the outter skin.

I used to run a full crossover box, but now just use the built in crosses in my amps. Both of mine are mounted under the rear seat. That's where my crossover used to live as well.

For my super tweets, I just put a filter in line to cut out everything below 9,000 Hz, so no need for the crossover.