OK, i`ve just had a look at my own 2021 GSA heated pillion seat and probed it with the multimeter.
If you look at the seat with it upside down and the open end of the plug facing you you have 4 pins, from L to R lets call them 1,2,3,4.
4 is brown and common to the other 3 pins, and i believe that to be earth/0V. For the following tests leave your MM -ve probe on this pin.
The adjacent pin 3 is power for the heating element and measures 9.7 ohm which corresponds with the front seat and works out at a reasonable ~20W.
Pins one and two i believe are signal/control inputs to tell the ZFE that you have selected heating either high or low power.
With the switch selected to one dot (lo heat) you should have a dead short between pins 2 and 4.
With the switch selected to two dots (hi heat) you have a dead short between pins 1 and 4.
The pins are quite recessed in the plug so you`ll need slim MM probes and three hands....
I used a small length of solid copper wire to slip into pin 4`s slot and used a croc clip on that to hook up to the meter so i had both hands free.
You say your TFT is recognising the call for heat - the `1` or `2` symbol next to the seat icon so it may be that your seat element has gone open - pay particular attention to the resistance between pins 3 & 4.
Doing those quick tests should tell you if you`ve got a simple fault with the actual seat or something more sinister further upstream.
Good luck.
ps if all that checks out i`m fecked if i know, you`re getting into the TFT/ZFE and all that electronics bollix and that`s way beyond me, i don`t trust anything with no moving parts....
