There is no sensor in the seat to tell if someone is sitting on it. Any dealer that tells you that doesn't know what they are talking about.
However, there are thermistors inside the seat that sense the seats internal temperature. If the ambient temp out is above about 55 or so, you may not be able to get the seat to heat up enough to feel it.
By the way, I am now experimenting with 4.7K ohm resistors on both front and rear thermistor circuits. It may turn out that I need about 5K ohms on the front and 10K ohms on the rear. It's a bit of a trial and error process trying to dial in just the right size that isn't too hot when it gets cold out, but not hot enough when it's 55 degrees out. So far, the 4.7K ohm seems to work fine for the front seat.