There are a few options. There are 4 point seatbelts that clip into the seat belts in the back so that they don't need a crossbar. 4 points are very uncomfortable. There is nothing to counteract the up tension of the shoulder straps, so it's constantly riding up into your stomach. You have to set the lap straps so tight to counteract the tension that they cut into you. You can't run 5 points with stock seats, because there is no hole in the seat for the 5th point. The reason harnesses are not DOT approved has to do with the release mechanism. It has to be a standard release mechanism so that anyone can come up to a crashed car and get the seat belt off an occupant to pull them out. My recommendation is to either put race seats in so that you can run a 5 point, or just rebuild your stock ones. We sent some off to this place recently. I think it was $69, and the fixed everything on the belt, and they put in a new strap.
http://www.safetyrestore.com/