Water Leaking From HVAC Housing Onto Passenger's Side Floor

I have a 2004 100 series and have just experienced water appearing on the passenger’s side floor. This is leaking from the HVAC housing. At the time this occurred, there was no water dripping from the evaporator drain tube.

I have removed the cover from the evaporator housing but this all looks clean inside. I ran a flexible curtain wire up the drain tube outlet and then went on a 1 hour drive. Water was now dripping from the drain tube and nothing inside. I made a return 1 hour trip the same day and all good.

Today with higher humidity I had water dripping inside on the passenger’s floor again AND water dripping from the drain pipe in the firewall. I think that I have a partial blockage at the drain tube.

My big question is, can I cut a 1” (25mm) access hole into what I believe is the “boss” in the housing located and easily seen from the vehicles interior (just above the carpet line). I believe that is in line with the drain tube outlet? My intention is to gain access to whatever is blocking the outlet, clear it and then bond a blanking panel over the access hole that I would cut.

Does anyone have an HVAC system out of a 100 series and could check what is on the inside of this “boss”? I will try to upload 2 photos of the area I am referring to. One of my photos has a white sticker with a cross on it which is where I would cut my access hole.

Many thanks in advance for any help. PS, I have researched this and found various suggestions but none referring to gaining access as I have asked above.