how can I access the area under cowl / vent

Sure, are the fenders off?

Here is what you do:

The cowl has its lowest spot just like the rest of the car so that water will exit any compartment of the car that it enters. There is always a pinched opening between two panels where the panels form a chamber and the opening is at the lowest point of that particular chamber. In the cowl chamber, the openings are about 1/4 inch tall and 5/16" long just above the hood hinge pivot shaft. I took a hole saw and enlarged the holes on each side to about 2.5 " in diameter. To do this carefully and not damage anything, you cut the hole so that the lowest portion of the radius encompasses the existing cowl drain hole. This way you essentially just include the factory drain hole at the lower edge of your bigger hole. You only cut thru the outer wall of the cowl chamber.

Then I used 140 psi air, vacuum and more air till the inside was empty. Then I ran a sand blaster all over inside the cowl plenum, blew it out for several more hours and soaked Eastwood's internal frame coating all in there big time.

Only couple pics that I have, you can see the holes are enlarged, and one pic when it was just in epoxy, circled in yellow: