If there's a lot of leaves it may be best to remove the drivers side vent and the heater box on the pass side. You will have some room to reach up into the cowl at that point, becareful of sharp edges. If you have a compressor use the blow gun in the vent in front of the windshield to blow the debris towards the two ends of the cowl where you could reach it. Dropping the heater box will also allow you to clean it out since the fan may have drawn leaves into it also.
You could vacuum it out also. Sacrifice a Shop-Vac hose by cutting the hard plastic extension from the end of the hose. This "new" end of the hose will be flexible and allow you to push it up into the cowl from the removed vent and heater box and slide it sideways along the interior of the cowl. This should remove most of the debris, even if it's a little damp. Use the smaller diameter hose (1 1/4") to do this.
Working from under the dash is a pain with lack of room and an uncomfortable position to work in but it's easier and cheaper than cutting the cowl open.
Another tip is since the heater hoses go through the firewall before terminating at the heater core don't disconnect the hoses at the core, this will create a mess on the front passenger floor with antifreeze. Drain your antifreeze out of the radiator, then disconnect both heater hoses from the motor. Run the lower hose from the firewall into a container. Blow compressed air in the other hose forcing the antifreeze out. Now if you disconnect the heater hoses in the car you won't have a mess.
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1968 Mustang coupe, Acapulco Blue, 289 2v, C-4, Power Steering.
Hopefully a 1969 convertible or Sportsroof (non Mach or Boss) for next Mustang project
Trying to find my father's 1973 Mustang Grande he bought brand new. 3F04F126773 last known registration and title was in New Jersey, 1982.
cowl cleaning out 
