[MIPS]

An oil leak that large should give a LOT of noise as the AC system vents. Are you positive the AC still works?

When you mention a T-junction though, I know some vehicles (my eagle included) shared the heater core in the same cavity as the evaporator and thus used a bypass valve to shut the heater core off whenever the AC was turned on. Due to the high heat and the mechanical nature they are famous for leaking.

Edited: one other thing. Are you POSITIVE it's not just condensation dripping off the evaporator? It is Arizona but one good rainfall is enough to crank the humidity up to 80% for an afternoon.