or handbreak or bonnet release catch. there is always a few places, just wire it all up and leave the earth until the end and then find a place that works.
just make sure its clean metal and it should be fine. try and get it to be on like a main part of the car, like the earthing block metioned above ( if you have one?!) or the handbrake, rather than like through a panel or a welded joint or something.