Unless you live in an arid desert where you can count on any amount of water evaporating overnight, power washing the bat guano inside the car is likely only to spread the offensive material around and result in a septic swamp. It will make it worse rather than better, which you will probably find out if you check back with the guy from the March 1st 2009 show later on Stump the Chumps.
What should be done with any offensive smells from organic materials is to attack the source with an enzymatic cleaner that will digest the organic residue that is the source of the problem. There are many brands readily available from pet supply shops across the country, and they really do work. First step is to remove the bulk of the offending organic material, scrubbing away any visible residues. Then you apply a thin layer of the enzymatic cleaner and leave it to work (if the appearance of the surface is importance to you, test a small inconspicuous area first to be sure that it doesn’t damage the finish). In the case of a sloped surface like the dashboard of the car, one should soak a cloth with the cleaner and drape it over the entire offending surface, re-soaking and re-applying the cloth frequently so that you keep the enzyme liquid in contact with the offending surface for a minimum of 24 hours or until the smell has disappeared to your satisfaction.
As to prevention of critter/insect & rain entry while ventilating the vehicle, I suggest buying one of the plastic window inserts that fits into the top of the window, providing a small rain shedding roof over the ventilation slit - I’ve not seen these with built in window screening, but that is easily resolved by getting a large sheet of window screening or other fabric, drape it over the outside of the whole window (or windows) and anchor it in place with long magnet strips. We’ve found that magnet strips salvaged from old refrigerator doors work great because they are very powerful. It looks a bit odd, but has allowed us to keep the heat from building up in our vehicle while keeping it relatively secure - very important when traveling and living out of the vehicle.
Can’t say I’ve ever had that problem.