the flap is under the windscreen scuttle, basically do the folowing
take off windscreen wipers, you may be able to get away with just the passenger side on a 156
take off that long rubber bulkhead seal from under the bonnet that goes from one side to the other
look ot the black plastic covers at the bottom of the screen (left pic) and undo the screws holding it on at the bottom, 3 or cross heads IIRC, then slide you fingers between the screen and upper edge of said plastic and pop upwards and it will come off, about 4 poppers hold it one. Move part out of the way.
Next, under where the wiper was there is a plastic cover, simply remove that (right pic)
and peer down the hole, if there is a flap visible when the recirc button is off, there is your answer, give it a whack with a long handled screwdriver or bar and pop it back open. Useless feature anyway unless you are in a 10 mile tunnel.
If not, reassemlbe and start looking elsewhere for your leak.
a couple of pics to help you find the target, off a 145 but pretty much the same.