if he does it to tamper with federal investigation - then i'd guess it'd be a violation.
If he does it to improve fuel efficiency of the car and remove unnecessary weight from the car ... it'd be interesting whether it is possible to bill the government for the service of carrying their stuff around. Or for not being able to use the car at all as having the GPS tracker attached the car may become unsuitable for your use. Shouldn't anybody whose property taken over and used by the government to be justly compensated?
Well you can bill the govt. but not sure you'll get paid ... ;-)
In general, and most people are surprised by this, you cannot sue the US govt in court. Well you can, but only in certain cases where it allows itself to be sued -- tort cases and some IP laws and even then you almost never get a trial by jury, only a federal judge.
So you'd have to construct this as a tort case and use Federal Tort Claims Act (FTCA) to your advantage. I suspect if you try that, FBI will invent some reason to arrest you or at least harrass you (interfering with an investigation, obstruction of justice, damage of federal property, etc etc...).
If he does it to improve fuel efficiency of the car and remove unnecessary weight from the car ... it'd be interesting whether it is possible to bill the government for the service of carrying their stuff around. Or for not being able to use the car at all as having the GPS tracker attached the car may become unsuitable for your use. Shouldn't anybody whose property taken over and used by the government to be justly compensated?