I suggest your radio is the "canary in the coal mine" and is displaying
another problem - unrelated to the radio.
If this were my car I would check the battery, its connections and the
alternator's output. The alternator uses diodes to regulate its output
(voltage regulator) and these diodes fail after a very long time, like
19 years! The diodes alone can be replaced.
If nothing is found then I'd put an amp meter on the radio's circuit to
check its (actual vs. specified) power draw. And unplug the power
antenna's power supply line to ensure its motor isn't the cause.