I doubt the dealership will go as far as checking the heated seat option. From my experiences with trade ins and others friends that I know, the appraiser takes about the 10 minutes to evaluate the vehicles value and rarely checks if each feature is functional. They checked for overall exterior and interior appearance, odometer mileage, and did a quick test drive around the block, then came back for an appraisal on the vehicle.

Here is what I would do, unbolt the seat and look under it to see if there is a connector that can be unplugged so it’s not getting power at all. I would take my chances on the dealership appraiser to not even check if it works. YMMV