The motors to adjust the seats may be ok as they are used for short intermittent bursts but there is no way I'd be wiring the heating element piggybacked off the standard wiring.A couple of months in one of them and no snags. If it was going to burn would it not have gone up by now?
It's dead simple to do properly :- heavy gauge from the battery through a relay using the ignition wire to switch the relay.
Also above when it was mentioned the 7.5a fuse blew so it was changed for a 20a if you replace it with a nice 8mm bolt that will then never blow it had a 7.5a in for a reason