Vehicle buttocks detection
you can use FSR(force sensitive resistor) to detect whether a person is sitting on a seat or stands up.
The working of FSR  is similar to LDR (Light dependent resistor the resistance of LDR would decrease if light falls much),in FSR if the applied pressure/weight increased the the resistance would get decreased,you can calibrate this resistance change in terms of weight/pressure.

fig(1) weight Vs Resistance and conductance


fig(2) circuit diagram- connection of FSR


fig(3) Output voltage Vs wight(response of circuit diagram fig(2))


 For more details http://www.tekscan.com/flexiforce/force-sensors-for-design       
  http://learn.adafruit.com/force-sensitive-resistor-fsr