Both Discos have a nice puddle of water sitting on the cargo mat after a car wash. Both have ladders. Always been a problem and keep forgetting to ask if anyone has found a good fix.

 

I've cut a large bungee cord & stuffed it behind the gasket across the top and it stopped water from seeping in around the ladder.

 

What Best said or if you don’t have a bungee cord some backer rod from Lowes or HD. Also you can adjust the rear door striker inward to close the door a bit tighter.

 

I’m not sure if this is your issue or not, but mine had too much gap at the upper edge of the door, so it couldn’t seal. I loosened all the hinge bracket bolts and then, with the door wide open, I lifted the left (now back) corner of the door while my wife tightened the bolts. This closed the gap at the seal (actually I got it too tight on my first attempt). It seems to seal fine now.