I believe it's still possible to be a fuse or other non-seat problem.
Judging by the schematic, it looks like the button gets its power from the dash fuse block, and the heaters themselves get power from the rear seat fuse blocks... The first thing I'd do is check under the back seat for the
HTD ST fuse(s).
(BTW, the mirror heat has nothing to do with the seat heat.)
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97 Riviera L67 - Desert Sandstone - Gutted AB Lid, Kool Blue AF, 180* SuperStat, Hogan 3" DP, ZZP 3.6" MPS; Sensa-Trac struts, Rockford-Fosgate 6x9's; Annoyance-Fixes: Express-Down delete, Full-time juice to Power Port, BRP-power to Glovebox lite, Brake-to-Shift delete, Console-compartment lite, Seat/Mirror Memory in any gear.
69 Wildcat Convertible, 430" QJet - Burgundy/Black - P/Win, P/Seat, P/Ant, Tilt, Cruise, Remote Trunk, Rallyes, AM/FM Mono!