No one will open the doors because why?
Well, you'll want to either build or buy crossovers for your tweets (if you don't have 'em already)... of course run wiring and set mounts. If all you're doing is adding tweeters to your existing setup then you're set. Otherwise you can look to upgrade your head unit, full component set w/ amplifier, subs, etc. Depends on what all you want to do as far as an 'upgrade' goes.