damp pro is great and will do wonders however if your tracking down an individual rattle its going to depend on what is rattling. What i would recommend personally because ive been in that same situation is Install the system first. If you're doing rewiring then leave whatever you had to take out of the truck out and try to locate the rattle with the system playing and the interior "out". That way once u find it/figure it out u can tackle it use what u need to get rid of it then use the rest evenly on the rest of the truck improving everything (you also should do a 2nd test once u eliminated the first rattle because theres a good chance there will be a second or 3rd or whatever that u didnt hear before)
going with the bulk pack of damp pro is a great way to go about it. I would also buy a sheet or 2 of the overkill (not pro). Its cheap and if you run into any plastic rattling that will help. You can also use it on the areas where the trim clips in