If only the cushion of the drivers seat is malfunctioning and the backrest is working well, the problem is to be found in the seat itself.
There are two heating elements in the seat, one for the cushion and one for the backrest, and these two elements are connected in the same plug under the chair.
Check the 2-cabled black plug under your seat for ground & power after the seat heat is turned on...I presume the control light in the switch is turned on?
greetingz & good luck
with a minor 600+k