Technical 05 Ducato,,, heater box drain?

[Ocwobio]

Hi all,,, just had the aircon on my motorhome re gassed, and now it is back in use, I notice a drop of water under the dash near passengers right foot. I reason it has come from the ducting someplace.In my day, they had a drain from what they called the 'plenum chamber'. I understand that heater boxes have drains as they sometime draw in rain in exceptional circumstances. So, how to locate the drain and poke it clear? The MoHo is a Burstner i821, but all scuttle/dash/heater box/ etc. is all Fiat as built. Access from above in the engine bay will be a pain, but what about from below? Ta very much for help! Cheers!! Dave