Search cluster test...solidjake has a youtube video of how to do it...that'll give you your voltage of the car when off and on.
Give the battery disconnect a try...but first, see if the cables on it are tight or not...shop might have disconnected them and forgot to tighten them up...who knows?
Upcoming NYC dates: Jan 8 and 27. See Facebook for other shows and PM me if you can make one...love to see you there!
Mangofy your E46!