Build an 89C52 based control for your bicycle seat heater that controls the duty cycle of the PWM to the heater driver transistor in reverse proportion to the amount of force being exerted on the pedals. The theory being that the need for heat to the seat will decrease as the rider works harder to propel the unit. In addition, if a generator is in use as the power source for the heater, this control scheme would lessen the load afforded by the generator making more of the riders exertion available for turning the wheels. With the generator another sensor will be required to measure the generator output so that the software can null out the percentage of the initial pedal pressure measurement that is going into the generator. Certainly such a system will require some calibration constants so don't forget to include a small serial EEPROM to store those between bike rides.
Michael Karas