AM radio to Bluetooth.. or other ideas.

You can just leave the stock radio alone and functional. If you want a really nice system, install a DSP(Digital Signal Processor) with as many amplified channels as you want feeding some quality speakers. The DSP's can actively cross over the frequencies to tweeters mid-bass and subwoofers. You can also bridge several of the amp outputs to increase the power to the speakers.

You use your smartphone to control the system and feed your favorites via bluetooth or plug in a data cable to stream Tidal or Apple play etc to the otherwise hidden system.