shouldnt be to hard, id imagine the head units would not have rca outputs so you simply would have to get an amp which has high level inputs (which many amps these days have for that reason), and connect the factory speaker cable to the amp, and your set. easy
In my BA XR8 ute i have had Focal 6x9 in the back and splits in the front and an alpine amp installed. Looks totally stock, no different speaker covers cut in. Seriosly thinking about a sub. I think it's Clarion or maybe Alpine that make a sub that is very similar in size to a play station 1. The magnet sits inside the cone and it is all self contained. I've measured it up and it would sit great on in the spaces cab. Current system is great on CD but when on radio makes me wonder why I spent the money!
I have a orion 15 inch sub run by a kenwwod amp in my xr8 sedan.
I sourced power fron the amp from the acc near the head unit.... spiced the rcas into the rear speakers so it can be faded in and out to my liking with the back speakers...