@Venom700 : The provided weather strip is 3/4x3/4 I believe so get something close to this size in marine grade.
Well by tailgate is really cannot be water proof, if you look how the tailgate closes tot he rest of the bed body there is a hole. I just recently tried to add some seal there but I have not had a chance to test it out yet.
Car wash is fine, problem is long persistent rain (or I would guess thawing snow but I live in Texas..) where the sponge seal get soaked and starts leaking. That being said I have researched wide and deep and talked with many people and I am yet to see a true waterproof solution. If waterproof is a must I would get SUV rather than truck (I don't mean to be ****, I am just honest).
Also when mounting I strongly suggest to get few vise grips and use them to secure the rail (and really pressing down on them) and then screw the clamps. Also the guide says to glue the weather strip as a first step but I recommend to first fit the top in a way it seals with tailgate, make a mark with chalk on both ends, lose and slide the top away and then put the strip on (using the the marks). That will ensure it is in the proper position and seal better with the top.