Do you get a red warning triangle in the dash?
I would start by getting fault codes erased. I dont know if it stores the fault and pops up the error as a reminder. someone else could chime in on that.
Also WRT to radio, you may have a 12v but do you also get a ignition on signal on a different wire? not sure how these are wired but IME radios can have a separate 12v in or out when ignition switched on. Just some thoughts, someone else might know more on this point too.
In no particular order:
1. Check running voltage. Use multimeter with everything off but engine running, then switch everything on (blower, high beam etc) recheck running voltage with MM. should get 13.5 and no serious drop when everything on.
2. Check ground strap and terminal post at battery, clean.
3. Check positive connection in engine bay, undo clean refit (with batt disconnected).
4. Check engine earth strap at engine block and chassis. clean and refit.
5. Check and clean alternator connections
But clearing fault codes would be first step in case its just a left over reminder. Ive also seen a smaller separate mini fuse board up under the side of centre console where it meets dash (just in case you were looking at a different one). Again I might be wrong on these points but just for consideration in case they help or give a lightbulb moment.
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