I have not worked on RX-8 seat heaters specifically, but I've worked on several other makes... And they've all been system voltage (12v.) I would be incredibly surprised if RX-8's were any different.
And no, USB voltage will not work. It's more than just voltage, by the way... You'd have to have a system that can provide the substantial amperage as well. :-)
2005 Mazda RX8 Shinka 6MT