Drain the cooling system and remove all of the dash, radio, glove box, speedo cluster top centre temp gauges and all lower air ducts. You also need to seperate the heater core housing from the air con coil and blower housing which you leave in place. There is two O ring seals on the pipes from the top of the heater core that go through the bulk-head. These might be leaking. I soildred these pipes because new seals were not available from Toyota are getting a reconditioned heater core / matrix.
It's a big pain in the butt job to do and it's not covered in the factory workshop manual.
I removed the front seats too, for more work room. Good luck!!!