This does not signal the airbag system nor is it a seat heater. This is nothing more than a pressure sensor to trigger the seat belt warning light for the passenger seat belt. You can test it by plugging it in, turning on the car, and pressing your hand down on the sensor with the passenger seat belt unfastened. You will see the passenger seat belt light start blinking in the center dash area. With pressure still on the sensor, you will notice that once you fasten the seat belt it will stop blinking. The 2000 4Runner does not have a pressure sensor in the passenger seat for the airbag. To directly quote the 2000 owners manual page 50, lower center of page, "the passenger airbag is activated even with no passenger in the front seat." That is to say, if triggered, the passenger airbag will go off regardless of a passenger sitting there. And if a passenger IS sitting there with no seat belt on, and the airbag goes off, they will be thrown into the explosion (instead of being properly restrained by the seat belt) and will most likely die.
SRS (the airbag system) stands for Supplemental Restraint System, meaning that it is a secondary supplement to the seat belt. The seat belt restrains at a certain distance and the airbag inflates to a certain distance within a certain time frame. Working together, they offer the best chance of survival in a straight on hit. The airbag by itself can be fatal.
So, leave it plugged in, and if you are swapping lower seat foams (like I did), simply pull the sensor up off the foam (it is glued on) and tape it back on the other foam (like I did using clear Gorilla tape) so it remains in the passenger location and continues to work as factory.
To sum it up, the blinking passenger seat belt light triggered by this sensor is Toyota's way of saying, "Hey Chucko, we noticed that you are sitting in this seat. That's fine, but we're gonna go ahead and annoy you with this blinking red light until you fasten your seat belt because if you don't, and you get in a head on, the airbag will go off anyway and you're gonna be out like a fat girl playing dodgeball, and you ain't never gonna make it to the 7/11. Domo." (See owners manual, addendum 2017, PP 672, lower right)
Cheers,
Justin