All suggestions so far are valid, interesting, and will work. BUT...from what I've heard over on the Fender Discussion Page is that the single best electronic mod you could do to the MIM is the installation of a J-Retro pre-amp from Vintage Bass Trading Co.
http://vintagebass.com
They were recently reviewed with glowing results in Bass Player and everyone that has one has raved about it's sound. It's a 3 band unit with passive capabilities. It comes completely wired on a Jazz tone plate with chrome stacked knobs, ready for installation. From what I understand, some basses require a little relief in the inletting to fit while others don't. You'll do a disservice to yourself if you don't check'em out.