Legal Question in Civil Rights Law in Washington

Almost for sure the results would not be admissible in any court in Washington or at the federal level. Additionally, you may be subject to criminal prosecution for placing the GPS on a car you do not own and actively tracking it. Harassment, stalking, and trespassing are the charges that come to my immediate mind. If the GPS goes into a structure, such as an attached garage, that may also constitute burglary. Civilly most of these apply, as does invasion of privacy.

Bottom line: Forget this idea ever came into your head. It will get you nowhere but in trouble. Separately, but in a related note, if you want to prove something along the lines of your ex-spouse's bad behavior, spying on them to get the information is most likely going to be frowned upon by the court commission or judge hearing how you obtained your information. The reason is partly because this type of behavior only further escalates an already divisive matter that needs no more tension brought it.