That red truck is of interest to them, and I can understand why. I think they might be calling JM a witness, due to him being on the scene that morning, in addition to something they didn't like about his vehicle that day, (I don't think it was about the tags, unless the prior owner was of interest, and/or possibly some of his actions that day).
Installing a gps on a vehicle to monitor the vehicle's movements, is considered a search under the Fourth Amendment. I think they have 10 days to get the unit on the vehicle, from the time of the warrant.
JM is considered a witness. He was charged with tampering and vandalism, which the judge dropped, and handed over to the Grand Jury. The Grand Jury, if they proceed on it, will be pushed to show their hand, by JM's atty.
LM said he thought it was about the 2:00 a.m. text message. I don't know if the text was the reason or not, but if I were LM, I'd have kept that to myself. JM could be called a witness simply by being the first person to walk into the crime scene.
Witness - Definition, Examples, Cases, Processes
To get that warrant, to put the gps tracker on the vehicle, a judge has to determine probable cause.
OSBA | Search Warrants 101
So, it seems, they simply wanted to keep track of where that vehicle was going, for the next 45 days, and delivered good reason to a judge. I wonder if JM was the only person who ever drove that vehicle?