Well, you got some interesting suggestions but now that you told us exactly what you want to do, you might want to hear about something more practical.
The suggested weight sensing switches would have to be fitted somehow to the car seat mounts, tricky and potentially dangerous if it involves altering the seats frame.
What you are looking for is a "pressure switch". You have seen versions of that under floor mats at shops where a beep goes off as soon as someone steps on it, thus alerting the shop staff (who might be busy behind the counter).
A pressure mat is thin and flexible and could easily slide under your seat covers, you just need to find one of the right size.
They could either work by sensing air pressure and the switch is a membrane that triggers a micro switch or they simply have some conductive foam sandwiched between the covers and the foams resistance changes to give you an output. The mat I have here is rated 1 Amp @ 30 Volts - you should be able to interface that easily enough to a digital input device.
You did not really want to "weigh" anything, just sensing the presence of a weight. It sent a few respondents up the wrong path so to speak :wink: