only think i can think of is the air pump/fan for the emissions. look underneath and see if its got a empty plugin. its below the brake stuff and intake air.

 

Its not been plugged in for a very long time, GM tends to make harnesses for all options then not use what is not needed. Example: All cars are wired for power seats, cars without power seats just have plug under the carpet and no fuse in the fuse box.