The tan wire off the injector runs to the PCM and is the negative pulse signal wire. The only reasons I know of to tap into that wire is for an alarm, a remote starter, or a tachometer. If you have any of those, then that's what the wire is for. If you don't have any of those, then the wire could've been there from a previous installation of one that someone has since removed but left the wire in place...unattached.