@Jay I got in the case with this today and ripped the dash out. Brought back memories/nightmares of the last time I removed one with a mate of mine on my old one, remove a leaking matrix.
As you said Jay, there's a rod on the on thr bottom left of the vent. The vent is solid shut. I managed to pry it open a bit but it seems seized.
I connected the heater centre module back up and tried to test the heaters with tue dash out but they won't turn on. Guessing one of the plugs thats on the dash is needed robget the heaters to turn on?
The car turns over but does not fire up also.
Again is this due to one of the electric plugs I removed?
Cheers for any input.
Ps - do you happen to have a spare working heater box at all?
Thanks