my 02 gtp had both seats heated, so it had switches (plural), same with my gxp
Passenger heated seat was definitely not an option on the '97 - '03 GPs, so he either custom did it himself, or he bought one of the aftermarket dual heated seat kits they used to sell.
Grand Prix Dual_Heated_Seat
Shame they don't sell them anymore... GF is always complaining that her seat is cold during winter
TL.. send me two switch panels and I can make you a dual. It's pretty easy.