Does your aftermarket kit include any wiring diagrams James?
The original switches are designed to operate on a timed basis, and not to operate at all if the thermistor in the original seat heaters go open i.e. when it is up to temperature.
It just seems a shame not to utilise the wiring in place in the car, and the added benefit of it looking OEM too
Switches
HERE or have a word with one if the traders on here
Brian