OK Wittsend,
 This is not as good as removing the fenders,BUT, If you remove the inner splash you can reach up and pull out the BIG stuff clogging the drains.
 I did this on a C. wagon I had and let me tell you I used a garden hose with a squirter nozel.
 I did this on a concrete driveway and was there for several hours. First squirting from the drivers side and then from the passengers side.
 As I was on concrete I could see the derbis that kept coming and coming from the drains each time I changes sides.
When you think you have it all be sure to give the back side of the fenders "Where all the junk is goiing" a good
power wash too.
 After you think you have it all out maybe then drive to the local car wash and drop a few bucks in quarters.
 From Pintony

Pintony,
  Thanks, I just remembered I have a pressure washer.  Hopefully it won't take the paint with it.

  I think there is a real market for a flexible, magnetic screen that would fit over the cowl. Something like refigerator door magnets with a screen attached and rubberized so as not to rust.  Two or three sizes should fit most every car.

  I've got one space in my garage and the Sunbeam Tiger gets it. The rest of my fleet (Pinto = #9, and oh, isn't the wife happy about that) are all outside with many trees on the property.  I could start a garden club with the "mulch" I pull out of my cars.

Tom

2 words for you Car Cover x-9

Well..., I got inside the cowl area today.  The good news the debris was less than I thought.  The bad news is the driver's side vent opening was rusted.  A few things seem strange.  One, the rust was part way across the bottom (that makes sense), but it was rusted more severely on the entire vertical edge closest to the steering wheel. That doesn't make sense. The rest of everything is fine. No rust issues.

  The other strange thing is my car has the front end pointed downward (slopped driveway, car backed in) and logically the water should have gone out the drain long before it migrated up hill and leaked through the rusted vent access.  I say that knowing there was debris under the cowl, but it was far less, and far less compacted than I thought. It should have drained. The nearest windshield drain hole seems too far away to provide the source either.  Nothing under the dash indicates that the windshield gasket is leaking.

  Anyway..., what a "joy" it is to work under the dash. Welding and grinding is the worst. The blood runs out of your hands, everything falls on you.  I had an old mattress pad and cardboard down and when I started to MIG in a patch panel - of course it caught on fire.

   I hate welding sheetmetal (at least with a MIG)!!!  It warps and pulls away. Clamping was not easy with the limited access. No matter how clean you get the old metal it still isn't clean enough and you blow holes in it (and to think the whole idea is to get rid of the holes). The welds come out looking like knotted rope.  I spent more time putting in a "L" shaped patch 3"x4" than I did making and replacing the whole front passenger floorboard.

  After that I'm really looking forward to the passenger side - not.

Tom

i did mine in the car with my hands and
 bent tooth brush which i bent with heat from a heat gun. Also a vacuum cleaner with just a hose you can bend the hose to get everything around the lip worked great but removeing heater , ect was work.......... .