Why is Coolant leaking from rear of 2003 Chevy Trailblazer

Heater core is under the dash. There's nothing with anything liquid in the rear of the vehicle unless it has an auxiliary heater. None of our reference materials show an auxiliary heater.

If this is a 4.2 inline 6, that is actually the catalytic converter that the evap drain tube is near. That would indicate a leaking heater core, also may be some evidence(coolant) down under the rh side of the dash where the heater ducts vent. If it is at the rear muffler, there is an auxiliary heater core. The front heater core would involve complete instrument panel removal. The rear core would involve removal of the right rear quarter trim panel, to access and disassemble the auxiliary HVAC module. In either case, the A/C would need to be recovered/recharged.