So what would a sensor cost that checks body heat in the back seat, and if

[Ilsa]

56. General Motors is working on this.

http://www.gmc.com/gmc-life/suvs/acadia-rear-seat-reminder.html

Seat Reminder does not actually detect objects in the rear seat. It works by monitoring Acadia’s rear doors. If either rear door is opened and closed within ten minutes before the vehicle is started, or if they are opened and closed while the vehicle is running, the feature is intended to activate.

Once the vehicle is switched off, Acadia is designed to sound five audible chimes and display a visual message within the instrument cluster’s Driver Information Center, which can help remind the driver to take a look at the rear seat before departing.

The feature is active only once each time the vehicle is turned on and off, and would require re-activation on a second trip. Additionally, under some circumstances, the system may activate even though nothing is in the rear seat. For example, if a child were dropped off at school without the vehicle being turned off, the Rear Seat Reminder would still be activated.