you're driving e34 right?
most leather seats have wires and heating elements ready. in that case you only need to install a switch and plug wires under the seat. only way to be sure if your seat has wirings etc. is to remove seat and have a look.
i think it's no use build in heating elements and wiring, rather buy new seats from scrapyard (why not upgrade to sport/leather/el. adjustable ones

). all e32/e34 seats will fit.