There's folks who have put speakers in the rear cargo area before, but that takes up alot of what little space is already back there. Most just put in better speakers, from what I know. The stock units are considered absolute crap by alot of the "I have to have a touchscreen and 20-disc changer and 50 billion Kenwood speaker" type folks, but the local country stations still sound just fine to me. Might eventually get some aftermarket units myself though, seems I need to turn the volume knob a couple notches louder for the same sound level, and I don't know if the speakers "blew" or if I'm just noticing some hearing loss. As well as the vehicle has served me for the past 9 1/2 years so far, I'd be a fool to deny that it was in fact designed to be the econobox Jeep that would help Chrysler out of bankruptcy and ensure they could have a fuel-efficient vehicle in the Jeep lineup(thankfully one simple enough to be worthy of the badge, albeit the last such vehicle) so they could continue making the Wrangler a full solid-axle body-on-frame behemoth for the faithful(plus that four-door thing with the Wrangler badge for those who only partly get it).
Anywhoo, sorry for getting off track....I like to be thorough, even at some cost. @moparnumber1
built an interesting system for his Patriot while he had it, and made the rear sound box so he could remove it if he needed to haul anything. @big_willis
might have done something with his sound system too, I don't remember exactly but his build was pretty sweet for a lowered Jeep. Just recently put it up for sale out there in Australia as I understand, too. But, I'm afraid most of the brainstorming ideas you'll get here would be from old threads of folks who have already sold their Patriots for something else, and don't frequent this forum much anymore. The forum itself isn't nearly as active as it once was, and even on the 7000-plus member facebook group, while probably 99% do SOME kind of modification, a full stereo build isn't too common. Might be one or two though. The common stuff is the cliche angry eyes, the speaker upgrades, painting the trim on the steering wheel and door handles, maybe a seat cover or the touchscreen stuff(which I personally consider a sacrelidge of sorts considering that's hardly how Jeep became Jeep, but whatever), and of course the lift kits, "what size tire can I run", roof racks, and of course LED bars and pods, and bumpers/trailer hitches.
Sorry for long post....sometimes there's alot to say so you know what you're getting into. Facebook group is "Jeep Patriot Around the World" if you wanna "have a look-see".