We do not have residencia documents,live on a finca with only parcel poligon address and an empadron with that address and own 2 spanish vehicles. As an eu citizen residencia is not neccesary. We both have nie's and I have a Spanish social security number and have worked here for a catalan construction company perfectly legally.
So you don't have to be resident to own a vehicle here.

Re the previous post what does mot "equivalent" mean? An unregistered(in Spain) vehicle requires a mot of it's country of registration to be legal. I am not surprised the local police are getting "shirty" I know of people who are driving uk vehicles that haven't had mot's or tax for five years. But they think it's ok as they have insurance. I am sure in the event of a serious accident the insurance company will refuse to pay out on the grounds the vehicle wasn't roadworthy etc but as long as they seem to be covered when they "get pulled" by the police then everything is fine!