The conversion has started..

[zaphod]

Mk1 heater switch has 4 wires.
2 wires are the supply to the switch internal illumination bulb.
2 wires are the heater feeds.
Not sure if the seat heater panels are in series or parallel but a check on your seat wiring will confirm this.
As the switches you intend to use are hard to come by I would be tempted to fit relay feeds to the seats.
Then the switches run at low current to operate the relays.
That could avoid burning the switch contacts and preserve the switches you have..
Never taken a Mk2 seat heater ECU or switch to bits so you will have to look at the yellow plugs on your loom to get an idea of the switch wiring.

John

Looking at the gauge of wiring on the switch plug and that under the seat (there seems to be a second row of wires unused on the seat side) there is some sort of high current switching going on remotely form the switch as is. I'd surmise that is what the ECU you speak of actually is, the relay and some sort of array of resistors to give the differnt heat levels