Do you want a functional roll cage, or a show cage? Functional bars are sometimes not the greatest looking unless you get them powdercoated, chrome plated, or painted. Also, finding someone to weld the cage down through the floor to your frame might be a little difficult to do as well. Who ever builds one for you will have to basically, take measurements, put the cage together, do all the welding, cut holes in the floor so that the cage will reach down near, or on the frame, then weld everything in place. If you want a show cage, there's places like Monkey Bars (did Summer's tib a while back) or even APC that could probably help you out. The show cages are either hard plastic or aluminum, and they just assemble with regular bolts and screws, and then bolt down into your floor.
If you're only worried about getting the harnesses put in, then you could take out your back seat, have someone put together some kind of mini cage where the rear seats used to be, and then bolt the harnesses to that.
Pro's: Weight savings, More room for stereo system, looks cleaner, able to bolt in your harnesses.
Cons: No more back seat for passengers
God luck Khara

: