Accessory serpentine belt size?

So I just bought the car listed above convertible. The belt squeaks bad. I replaced it with a new one. Still squeaks. I replaced all pulleys as well. This car has mechanical tensioner. Just the pulleys were replaced. Found that the idler pull and tensioner pulley touch. So the belt touches as well. This is were squeak comes from. I took it to BMW mech. He said belt is routed wrong is why pulleys touch. Rerouted as described now the belt does not fit. My research turned up a different belt part number at BMW dealership. I purchased it and it is too short. Once installed best I can now there's no room for the top pulley and belt is tight as it can go. However my squeak is gone. What did I do wrong?