Coolant leak rear of heater core??

[Wallabee]

My 1990 535is 5 speed Australian spec vehicle is fitted with a Behr IKHA heater unit. I have a problem with coolant loss, with coolant leaking out the drain holes above the gearbox in the transmission tunnel.
I spent a day last weekend removing the console and heater cover to look at the heater core, expecting to find a leak that would be draining into the base of the heater box, through the rubber grommet drains and out of the drain holes. But the core was bone dry and there was no sign of coolant in the front of the heater box under the core.
It is item 19 in the picture attached that connects the drain points in the front of the heater box to the drain holes and these are dry. So I am assuming the coolant is getting to the drain holes through the other drain points, arrowed in the picture attached.
My questions are; where do these drain from and where would a leak be that gets to these drains? How do I access this area to find the leak?
Any help will be greatly appreciated!
Colin, Melbourne Australia