Wiring route question for heated seats

Hi,

I'm going to re-cover my seats with a kit from Exmoor trim so I thought I might as well put in the heated seat pads at the same time.
I can run the 12v supply from the battery box to the seats so that's fine, but I also need an ignition switched supply for the relay.

What is the best route to run a wire for the relay from behind the dash to the seat/cubby area? I'm thinking of putting in a carpet set too, so could i run the wire underneath the carpet that or is there a risk of overheating?

I've tried searching and googling but haven't seen how to do this!

Thanks,
Simon

just stick it through the bulkhead down onto the chassis rail next to the alternator/starter wires and back into the seat box through the grommet , if ultra paranoid use some split 8mm conduit