I fitted a single heated seat heater in the rear of my T6 for my daughter. I got Titan to insert it when they made my Titan bed and then wired it up myself afterwards. You can buy Seat pads such as
this one on Amazon or on eBay. The actual one I bought is no longer available but it looks identical to the Amazon one. It included the 2 way switch for low and high heat. I fitted one on the seat and one on the seat back, and extended the 2 sets of wires form these to feed them through the side unit on to the ignition side of the van battery via a 4 way plug and socket. I put the switch beside the control panel as it was accessible from her seat, and I could also see if it was on via the led indicator from the driver's seat.