It would work like this, you would wear a sort of an air-tight plastic diaper that handled the important genitals appropriately. The Diaper would then hook into the car seat by way of a tube emerging from your fly. During defecation you would push a button that created a suction effect within the diaper as to help with cleanliness. It could even have a built in wash cycle, considering the diaper is air-tight.

Afterwards there are two option of what to do with the waste. One, the easy more friendly option would be to keep it in a tank that could be emptied at truck stops and gas stations. Or my preference is a cannon built onto the roof of the car, which the passenger would control, so when someone cuts you off, or you drive by a McDonald's you can let them know how you feel without the noise pollution of a normal horn.

-Colonel Muffins