Most sensors detect some sort of voltage change. To enable the passenger airbag to be on all the time, you could place wire taps on the two wires going to the sensor and take a reading with a volt meter with the seat empty and then with someone sitting in the seat who is heavy enough to activate the airbag sensor. Then you could add a resistor(you would have to calculate the resistor needed) between the taps on the two wires that would send to correct voltage to the computer that would activate the sensor.
It may be even simpler than that if the sensor is just an on/off contact switch. If so, then you would only need to splice in a piece of wire between the two sensor wires.
Basically, you are tricking the computer.