Thanks for that Kevin - I've had a good look at that and it makes for interesting - and initially shocking(!) reading.
Reading a little deeper, this is as close to a factory set up as you can get, right down to connector-level.
There are two massive variables in his costs. The first is $340 for upholstery labour.
I have been quoted $200 to fit my kit and the second is the kit itself:
Dennis uses OEM seat pads with the heating elements built-in, at a cost of $600 in total.
The remainder of his costs are $190 for OEM switches and relays and the rest in connectors.
My Heating kit is costing $90 with $200 to retro-fit to my seats. If I stick with OEM switches (relays are part of the kit) then I'm looking at approx. $100 in Porsche parts with a max of another $50 in cabling etc. from Radio Shack.
I'll keep you informed.
