Q: Leak from under dash

Hello. It is likely the heater core causing the leak because it is on the passenger side and when they leak the passenger floor becomes wet. It may also be a heater core gasket that is leaking but the heater core itself is the most common cause of leakage. But when the heater core is replaced it is a good idea to replace the gaskets as well, vice versa. I suggest replacing the heater core and the heater core gaskets.