i have seen some real stupid installs in my life. like e.g. this guy who has been in business for around 20 plus years was installing an alpine h800 processor for the first time and running it active with PS 165. he was running the passive x-overs in the active system just because he had it in his mind that he will blow the tweeters. the guy putting in the system didnt know what and why he was getting that system (he spent above of 200k on it), he wanted the volume to reach maximum at 15 from the headunit.... the setup had 2 ps165 (front and back) alpine 910 dvd, h800 and rux 800, focal basstube and the speaker amp being used was some mtx. center channel was again alpine. the installer didnt know how to get the 910 to work on Ai net with h800, the buyer didnt know what kind of sound he wanted and the seller was just robbing him blind.
but there are some real good installers as well. you teach them one thing and they remember it. it always pays to go to an installer that does work on expensive cars. i have such a guy but even then i just ask him to run the wires and leave them in place so i can position the tweeters myself.
because we use the factory location for the midbasses, in a two way system atleast we only need to worry about positioning of the tweeters alone and even there you can find starting point by searching online for your particular model.
As car guru Xulfiqar keeps saying, you can be a better mechanic 100 times over the actual mechanic because you have the knowledge they refuse to seek. i say just get the wiring done and do it yourself. there is nothing more enjoyable than that..... ok well maybe there are things more enjoyable but its funnnnnnnnnnnnnnnnnnnnnnnn 
