The matrix is easy to get out, with just the lower dash removed, that's how I do them, just withdraw whole heater box, grind off the rivetts holding the lower dash support bracket behind ash tray area, pull straight out, screw bracket back in afterwards.
Are you sure it's coolant getting inside the car and not just water, a very common place to leak is along the bonnet release cable where it goes through the bulkhead, the panel that goes across the front infront of the heater motor fuse box etc has a drain outlet directly onto this cable, whose grommets rots away, so water come sin adn drips off the brake pedal onto the floor, that's why drivers side floor always rots!!
tabetha