I just use a water hose and a old speedometer cable on the 300D. The speedometer cable is used to make sure the drain is "rodded" out and then just wash everything out and through with the water hose. Also, inside the front fender wells at the back I cut the bottom of the "drain" lines out where it is now just a open ended "downspout". This insures that everything flows out freely. The drain piece was severely plugged with dirt and debris prior to cutting the bottom and washing it out. I don't run with any drain plugs anywhere in the 300D. I want that water to drain out to allow these areas to dry. Obviously, I don't have to worry about salt effects.
'85 300D - no rust here - 220k miles