[Mgs] clearing the drains in the heater system

[Simon Matthews]

My recently acquired MGB is leaking water into the passenger footwell
(it drips off the panel under the dash). I assume this is due to water
entering the heater vent in from of the windscreen and leaking from
there into the car. I seem to recall that the heater air intake should
have some water drains that need periodic cleaning, but I can't find
instructions on doing this.

Can anyone help me with instructions on how to clear these drains?

Simon