There should be a diagram under the hood that shows the treading of the belt. If this has been removed or is not legible your can try the GMC website for Buick's or if you google the topic you should be able to locate a diagram.
Where Do I Find A Diagram For A Serpentine Belt For A 2006 Buick Lacrosse If The Diagram Isn't Located Under The Hood Or In The Manual?
