A couple of updates. I made some brackets to put in a pair of 1992 BMW 5 series front seats, which attach to only the stock seat mounting bolts. They're leather, and full power, which only take 2 wires to hook up. I haven't finished the wiring yet though. Right now they're just attached to a battery jump box that I have in the back seat. LOL
front inside bolts:
front outside bolts, with a small 1.5 inch extension piece welded to the seat:
rear mounts:
I used one flat piece all the way across, just because the stock Volvo mounting spots were narrower than the BMW rails, and so the forces would end up bending smaller brackets. This way, the force all sits on top of that flat horizontal bar, and the bar just has two small tabs going down to the bolt holes. Nice and simple, and the only downside is that it cost a couple bucks more in material. No biggie.