LOL! He obviously has some trust issues.
You are overthinking it though. He is not some creepy, sinister, villain in his lair with a cape like Count Dracula lmao. He is just a guy who's been cheated on, who wants to outsmart anyone who might cheat on him in the future. He isn't directly trying to disrespect you- rather, he is just trying to prevent the possible disrespecting or cheating on, of himself. He isn't outright saying he thinks you are cheating -just that he is afraid of the possibility.
With that said, if you are on totally different pages, perhaps break up.
HOWEVER, if you are ok with the surveillance, and don't cheat on him, then I bet he will lose curiosity pretty soon and stop assuming you are a cheater.
But if you continue to ask him to take the devices out of your car, its going to continue making him think you "have something to hide". Thats kind of how it works. If you really want to show him that you are trustworthy, the correct move is to stop caring at all about the device in your car.