Replace the heater core. It's the only real way to do this. Hiding this huge box and trying to find a way to power it is going to take more work than just doing the job right.

 

I have a new hope.

i found a leaking, almost broken hose just at the entrance to the heater. Hopefully coolant leaks along through the firewall. as the floor is just a tad humid and in no way wet.

Anyway, yes if needed I’ll replace the heater core the proper way.

 

It can certainly get into the passenger compartment like that. Good find.