IIRC, you need not only the harness extensions / connectors (they may not be present if you have a lower trim level like an SLE). But you also need to activate the programming for them via a high-end scan tool, like a GM Tech2 (in fact, it may be the only tool that can do it.)
Finally, you'd need new driver / passenger door modules (DDM), because that's where the heater controls are located / wired to. And those have to be programmed, too, if new.
Depending on how badly you want them (and I love mine), you may want to explore retrofitting an aftermarket system to them (e.g.; Webasto, who I think had the contract for the OEM). You'd take their switches / wiring / relays and splice into the connections on the seats. Wiring diagrams are available on this site, if you need them.
That would save you all of the programming, and you could mount the Webasto toggle buttons unobtrusively anywhere you like (they're small & round).
The only issue I see here is that the GM controls have a 'seatback only' button, whereas your customized system would probably only turn both seatback / seat on at the same time. But I know Webasto has systems with 3 heat settings, which the OEM system utilizes.
(fair warning... Level '3' will get *hot*, after an extended time. Great for MI winters)