Are the speakers in the pods on the A pillar in a Mach 460 system tweeters or are the mid's and hi's? I would like to just replace these speakers to improve the hi's now that I have a different head unit to go with my 12's in the trunk.
LOL, best buy told you its not possible? Bunch of Dorks. The main piece of equipment you need is a line out converter which you splice into the data wires going to your rear mach amps. After that...install amp and subs and viola! You got bass! Do a search to find more information on installation. The truth is out there brotha!
I got one 10' sub in a thunderform box in my trunk pushed by a Soundstream amp. It really makes a difference with the bass response.
The A-pillar speakers are actually wide-range deivers, not true tweeters. They're very similar to http://www.madisound.com/cgi-bin/index.cgi?cart_id=5056653.2115&pid=102
If you want to change them, I'd suggest going to a full 2-way setup using a midwoofer in the door, and a standard tweeter in the corner. Most people won't know how to properly tune a widerange/fullrange driver. If you'd like to learn though, visit www.diymobileaudio.com