Try to keep them out of the doors. They need to be in a dry place that is away from other wiring, and as close to the speakers as possible optimally. Kick panel locations work well if the crossover will fit behind them.
Some cars have interior pockets on the doors (for maps or books or whatever). This is where i've put mine in both cars and it works quite well. You can drill a small hole in the bottom of the pocket (out of sight) like 1/4" or 3/8" in diameter and get all three sets of wire through.
Some crossovers are much larger than that though so it just depends on the size i guess.
best place is usually behind the plastic kick panel fascia below the dash board, in front of each front door, or up behind the dashboard tucked out of the way and wire tied there with plastic zip ties.
that's out of the way and dry, and nobody is going to kick them, sit on them, or hit them.