If you just want a switch that can switch between normal operation and always off that is easy. It would take a few minutes with a dremel a 50 cent rocker switch and a soldering iron.
Any car stereo shop can do it for you if you don't have the skills or tools to do it yourself.
"when my cell phone works perfectly fine and always has the latest maps for free." *
* free except for the cost of the data plan to download them.
I totally understand. I spent $300 a couple of years ago to put a 2 DIN android based stereo into my Jeep (which I'm now replacing with a used...
I plan to eventually upgrade my panel to 200A (already have a panel, just need to get the electrician and the cash to pay him)
But I'd be doing it more for other reasons (want a hot tub)
My short term plan is Chevy starts charging at 7PM when the rates go down, LEAF starts charging at 10:30PM...