I would suggest you check the heater control valve... often it goes after a few years and coolant will spill down into the carpet. It is possible thaqt the coolant spliied to the point that it found its way to the rear... slow drip.

Nextly, if you have functionaql A/C the expansion valve and copper piping area needs to be well insulated to avoid condensation and hence dripping... thisw is also located on the passenger side under dahs area, beside blower motor.

Enjoy!