If your vehicle is showing passenger side floor mat is wet or stained with coolant, here are the most likely causes and the parts you'll need to fix each one.
The heater core is a small radiator inside your dashboard that provides cabin heat. When it corrodes or ruptures, engine coolant leaks directly into the HVAC box and drips onto the passenger floor mat.
The hoses or pipes connecting the engine to the heater core can leak at the firewall joints. Worn O-rings or improperly clamped hoses from recent maintenance can allow pressurized coolant to seep into the cabin.
Some vehicles use a heater control valve located near the firewall to regulate hot coolant flow into the cabin. If the valve body or internal seal fails, it can leak coolant down into the passenger footwell.