Cowl Cleaning

I'm redoing some stuff under the dash including the heater box, wiper pivots, and vent boxes. While reaching through the cowl vent opening in my 68 Dart, I noticed about 1/2" of dirt, dust and debris. This obviously needs to be cleaned before I bolt everything back together.

Do you guys have any good ideas for cleaning out the cowl area. The car is already painted, so cutting the top off is out oft he question. Any ideas???

Thanks!