"Check the battery pan on the firewall". Hmmmm.
Other than a 2000 VW Passat, I'm not sure which vehicle you are referring to. The MK1 Audi TT convertible battery is in the "Front" of the engine compartment, not at the rear of the engine compartment where the firewall is located.

 

In response to the "Original thread". it sounds like it could be your heater core. The heater core (with the exception of a right hand drive), is located on the passenger side, behind the glove box, attached to the firewall. It's not uncommon for these to go bad.

 

Did you ever locate the drain holes? I have a wet spot just beneath the pedals of my lhd roadster. Never been able to find the source. But I know I can smell the antifreeze of the waterjets in the wet spot