A "hot" topic I'm sure

My bus has the lovely Comfort Seats fitted, 4-zone climate... but no heated seats!
So.. I know I can spend 2k+ and get an OEM look fit or spend ~£500 and have a little switch somewhere and not look OEM...
What is actually involved in the OEM fit? Seems a lot of money!!
I think I will need a heating element + fascia switches, but hoping my seats might have some of the other wiring and control modules in already?
Anybody have any insight?
Cheers