Some car radios have a reset button somewhere on the front panel, often inside a little hole that you have to poke an unbent paper clip or something similar into. If yours has that, try doing that reset operation.
If all you want is a plain radio, they're dirt cheap these days. A few years ago I upgraded my motorhome radio from the original one (with its cassette player) to a new one that cannot play cassettes but does have an AUX in jack and bluetooth connectivity. The new radio was all of $20 at Wal-Mart. It's certainly not superior quality, and its LCD display is sometimes a bit flakey, but works well enough for my needs.