You need to post your vehicle(s) in the garage so we know what year/body style you're talking about
http://ecomodder.com/forum/emgarage.php?do=add
Whether it's going to work or not will depend on whether there's currently attached flow down the rear glass onto the trunk lid. If there's not, and an entrained bubble is contained at the base of the glass, changing the current trunk shape might actually make the situation worse.
Tuft testing is a good idea.