passenger side? if so it may be your heatercore. It may also be a bunch of other things, maybe a cracked head? I think that would be a lot more obvious though lol. Im sure someone else will chime in with more helpful hints.
It's not easy to do a freeze plug at the back of the motor. The trans will not have to be removed if it is the freeze plug in the head, but doing so would probably make it substantially easier. And yes, I mean pressurizing the cooling system. There are adapters that go on the radiator cap to which pressure is applied, and while the system is pressurized you will be able to find where fluid is leaking out. Before doing anything or getting stressed about possibly having a lot of work, pressure test it first. Eliminate guessing wherever possible.