[lsversaw]

No new responses to this thread, so I'll add mine.

Instead of a tablet, use your phone, because it's already got an internet connection that's available anytime you've got a phone signal. (Yeah, this setup could use a lot of data, so look at your phone plan and try to get a large or unlimited data allowance.) Your phone also probably already has everything you want except satelite radio, and you might be able to find an app for that. For regular FM radio, there are websites that give you access to almost any broadcast radio station in the country. Get a mount that puts your phone in a secure, easily accessible position. DON'T GET DISTRACTED WHILE DRIVING.

And...your phone is already set up to manage simultaneous music playback, navigation, and incoming phone calls. It knows what you need to hear at any given moment and will mute the other sources.

Side note: All of our phones have an FM tuner built in, but the service providers disable it to force us to use the internet connection. Which uses the "data" part of your phone plan. Which gives them more revenue. Rascals.

Get a "car control app" that simplifies the screen interface in a way that makes it safer to use in a car. If you want examples, let me know and I'll post some examples here.
BTW, Google has, or will soon release, a version of Android Auto intended for applications where the vehicle doesn't have an Android Interface installed. Perfect for us who have an old vehicle and a relatively new phone or tablet.

Then install a stand-alone external amplifier. You can get a good, name brand, highly-rated 2 channel amp for under $100.

Connect your (securely mounted!) phone to the amp with a cable that connects the headphone output (it's line level, or close enough) on your phone with the RCA inputs (not the speaker-level inputs) of the amplifier.

Connect whatever speakers you can afford to the amp. There are a lot of threads here about speaker selection and placement.

Play and enjoy. It's simple, relatively inexpensive, and provides great sound.

With some modification, you can also use your original radio for volume control and turning the amp off and on. I recommend this because it's safer than using a phone or tablet screen for volume control. If you want to know how to do this, let me know.

Yes, you could substitute a tablet for the phone, but you'll have to buy a tablet with a phone connection. You'd keep nav, and music playback, but your incoming calls would go to your phone, and would not cause music or nav to mute when appropriate.

I welcome comments from others in either support or opposition to this idea.