That single two pin lead under the seats is most likely for the seatbelt latch. On seats equipped with heaters there would be three two pin connectors, one for the seatbelt latch and the other two for the heaters. There are two heater leads because the seat cushion and seat back have separate elements.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.