I will once again propose a solution to the emerging incidence of children being left in vehicles to be injured or die, by caregivers distracted by incidental events.
This is a LOW COST and VOLUNTARY solution, and it WILL work to reduce those deaths and injuries.
The solution is to install a set of wiring switches and lights and alarms.
The use of those is dead simple.
The system is armed by a key, with the arming switch placed in an inconvenient location, NOT accessible to the driver. The truck, or under the hood in the engine compartment.
When the system is armed, when the ignition is switched OFF, the system is triggered. An audio alarm sounds in a voice that reminds the driver to check for a child in the vehicle. A button switch attached to the seat or near the child is in is energized. that button must NOT be accessible to the driver from the driver seat.
Once the driver door has been opened, another audio signal is used to remind the driver to check for a child.
Once that driver door has closed, the button switch near the child seat starts a timer of 20 seconds. IF that button has not been pushed within 20 seconds of the driver door closing the vehicle horn sounds and continues to sound until that button IS pushed.
No complicated sensors needed, a LOW COST and effective solution, VOLUNTARY.
The vehicle operator has the choice of disarming the system when it is very unlikely that a child will be in the vehicle.
Once a child is known to be likely to be in the vehicle the system can be armed, with a KEY, that the driver may or may not have access to.
For instance a younger driver, using parents car, may not have the key to disarm the system. A parent, owning the car would have that key and COULD disarm the system.