How do you prevent rust damage inside a cowl, since it's welded together and you can't get inside to clean it out and repaint it? Is there anything you can pour down inside it to prevent rust?

Dave

Used motor oil. Make sure it hasn't rained in at least a week or you're sure it's nice & dry in the cowl area before putting the oil in. Expect oil drippings under the car for a few weeks following. If you don't find drippings, add more oil.

Drop the vent housings and clean all the junk out. Make sure your drains are clear. Then as mentioned use some motor oil

So pour motor oil into the arrea where fresh air enters the car? So you enjoy the smell of used motor oil enough to want to smell it while your driving your car?? After cleaning out all the debris & blowing the cowl out throughtly with compressed air I've always taken the normal spray nozzle off a can of black spray paint & found a spray nozzle that accepts a straw like comes with carb spray, then bomb the inside of the cowl with that.. You can fog all different angles, I use a couple cans again you want it to be dripping out of the drains & sometimes it even seeps out of the pinch welds...

The best way I know of to keep mopars from rusting is to:

a) Keep all factory drain holes clear of debris and moss, which I would imagine is very common in your area.

b) Keep the car inside a building with a floor that does not sweat.

c) Wax everything...