got the stock radio still? aftermarket adapters and such can cause odd ball crap too.
dic for volts is not really accurate, when they tested the load they can see volts car off. and on. on is the alts volts its putting out.
my regals battery also went really weak, same day the tahoes wend bad. i took the battery out and charged it for 12-15 hrs. then put my solar charger on it cause it sits all winter, last night it was like zero out, it fired right up. been sitting a week or so.