My rear passenger window will not roll down. Could it be a regulator, or switch?

It could be either one of those things. Pull the door panel off and use a test light to check to see if you are getting power down to the regulator motor when operating the switch. If you are not getting the power, then suspect the switch or wiring. If you are getting power and the motor is not operating, then you need a new regulator motor.

This is all assuming you have already checked fuses (though I am not positive if we have separate fuses for each window on our cars as I have never had an issue with mine that warranted checking).

Thanks for the pointers. I will check all of that and get back to you.