I used a car cover for a few years on my sd. Cheap one...not waterproof. It just encourages the water to flow off the car.
My gasket was hardened and cracked. I finally pulled it and put another in. I had to rebuild the rusted metal a bit with JB weld and fiberglass. The new gasket went in with a cord and dish soap. It was easy. Not sure about a 190d.
The only special tool I bought was two of those double suction cup handles from harbor freight.
It is so worth not having the lake in the trunk. Just one afternoon.
Bought the OE gasket from pelican.
My post
http://www.peachparts.com/shopforum/diesel-discussion/344462-replaced-window-seal.html