Is there anything preventing you from installing a kit from eBay...they have some nice sets that friends of mine have installed w/little difficulty and the elements were quite effective. You would have to router the console for switches or you could place them on the sides of the seat. Its only a few wires for install, one of which is a 12v connection so you could splice it into the harness. My w124 came with the heated seats so I have no clue about installing it. Just thought maybe eBay was a good idea for a moderate price but I'm sure this option was already considered as I believe it is fairly obvious.
If that isn't feasible, I would definately look for seats with the elements already installed and selling off your seats or keeping them as extras, since taking off the upholstery and wiring in the the elements would be a pain.
Best of luck

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2004 Chrysler Crossfire