The spoiler is not vacuum powered. It is actuated by an electric motor in the rear hatch. If it goes up all by itself then your shop has jumpered something they shouldn't have. You might want to ask them to check that wiring again. I recall someone had a similar problem with the spoiler when they installed a new head unit and posted here about the problems he had. If you do a search, you will probably find it.

The CEL could be a bunch of different things. Take the car to a dealer and ask them to read the diagnostic trouble code(s). Chances are one of your other mods has made the car's computer unhappy with you.