for best sound, you should mount the tweeters near the mids. i think the rule of thumb is no more than 12" but i may be wrong. the factory location (dash) is terrible for a set of tweets. it is not hard to mount them into the door panels. i haven't done it to my neon yet due to the weather, but here is a shot of one of my old cars with them mounted to the door panel. just a simple hole (i actually did it with a dremmel).
this is a shot of my other car which came with a much better mounting setup.
as far as the crossover, you can mount it anywhere you like so long as it is protected from the elements (water).