*Batman*

I would put it in the passenger footwell under the carpet. If you put it behind the radio you will have to remove the head unit if you ever need to reset it.

Every vaistech unit I have ever owned has needed a hard reset at some point including the ML+.

Because I put it in the wheel well, I was able to do the hard reset myself in 15 minutes. If it was mounted in the back of the head unit I would have had to take it in, or spend several hours.