These cars are sold with data plans which last quite a few years. What model year is your Blazer? I think that my Equinox has app access for 3 years and maps / google assistant for 8 years. I've tested tethering with my phone and it works with that, so I have a path forward once the built-in subscription lapses.

This is the one that I saw: https://evplay.io/shop/ev-play-lite-gm

It's kind of expensive, and there's a non-zero chance that GM does something to block it.

2024. I refused their privacy policy, so that might be why I'm getting nothing. I don't drive much so I'm worth more to GM than they are to me.

If GM tries to block it there are a number of ways a lawyer can fight back and likely win. The Magnuson Moss warranty act was historically written about car radios for starters. There are other consumer protection laws as well. You need a good lawyer, but I suspect they will take the case for the expected gains in the return lawsuit. If I were them I'd get a lawyer to write this up in a "white paper" - It would be a few thousand, but it is also something GM will likely see if they think about doing anything.