normally a squeaky belt is either loose, glazed, worn or misaligned.
do you have one belt or more? if you have air cond. is it on a seperate belt?
i would determine which belt is sqealing and check the tension. i belive its a 1/8-1/4" play in the main serpentine belt but not positive.
you say the belt and tensioner are new, maybe double check to see if its the correct belt? if the belt is one size different it could cause that as well.
if all checks out, you say you tryed belt dressing and its worse? belt dressing makes the belt "stickier" so it wont slip as easy. if it makes it worse, it might be a pully.
i use an old cut garden hose to locate noises, 3 feet or so. hold one end to your ear and the other scan over the motor untill the squeal is loud and it will pinpoint where its coming from. then you can determine better how to fix it. but be CAREFULL watch what you are doing with the engine running and the hose near the moving parts. hope some of this helps
lego