There is small drain pipe for the condensation formed on the cold evaporator in the middle of the black plastic heater/aircon casing which then drains out under the car. You therefore need to block up the crack/hole shown in your photo that has been broken/formed in the side of the plastic casing, so that it once again only drains through the correct central drainage outlet.
I guess the hole was caused by an accidental impact perhaps from someones shoe/toecap!
To replace the heater casing even using secondhand parts is a big job as the whole dashboard/ heater assembly would have to come out to remove it......far easier to block up the hole by gluing/bonding on a plastic repair plate/ waterproof amalgamating tape or similar.
Diagram of heater casing:
RealOEM.com BMW R50 Cooper Actuator, heater and air condit.
No.7 here is the condensation outlet hose which sits underneath the heater housing on the centre tunnel:
RealOEM.com BMW R50 Cooper Housing parts, heater and air condit. 
