I installed the crossover between the plastic door skin and the door panel. I recall removing a little bit of the foam barrier to make room. Cut it out with a blade in one small section. Then, just a strip of electrical tape to hold it in place until the panel was replaced. The crossover signal is from the door signal, then I ran a fresh wire down from the tweeter. The original tweeter wires are disconnected, as is the center channel.
For the tweeter, I used the stock location. I grabbed a $0.30 metal bracket from home depot that had various holes drilled. I just snapped the end off for each side, which had two holes ~.5" apart. One bolted to the stock tweeter location, the other side to the tweeter, moving it out a hair so as not to interfere with the dash panel.
The grommet in the door jamb, to run the wires through, is easy to remove/reinstall. So, pop that out. Put a stick/screwdriver through the center of it, then tape your wire to the stick and pull through. Then reinstall. I believe on the passenger side, the pass through hole is obstructed, so that's the only real challenge..but do-able.