GMT-800. I know a few guys have done the conversion and it works just fine. The heated seat portion is a bit more involved since 01-02 have a temp sensor so that has to be essentially grafted onto the 03-07 heating element or you can adapt the 01-02 element (not available new anymore) to the later style cover/foam.
I've seen the late seats put in, temp sensor retrofitted, and them work just like the OEM installed it.
As for connectors, I have a 05 LLY cab here that has the OEM required plug. I'd like to keep my 02 cab harness in tact so first step is to count wires and see if I can retrofit the 01-02 seat connectors to the 03-07 seats.
With the Katzkin kit, they have their own switch so I am going to likely try and 3D print a new insert to take the place of the OEM 01-02 heated seat buttons which shouldn't be hard.
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Later style meaning 03-07 with integrated belts. Those foams are available and the covers are available from Katzkin whereas it appears back foam is non-existent for 99-02 style seats.
If my back foams were good I would be keeping these style seats but unfortunately both my back foams are obliterated. The only good foam is the passenger side bottom haha.