U wont need a new HU you can get a (damn can not rememebr the name) but what it does is you wire it with your rear speakers and then it will give you preouts...NOT the best way to do it but it does work....

Best thing to do is go talk to a local shop....Of course take everything they say with a grain of salt, they will push their products and tell you its the best, and then next month they have something new that is the best

Also get a budget and stick to it. You can get a nice setup for under 500 bucks including HU and interior speakers...That is NO amps and NO subs....But it will sound nice...