My inlaws are experiencing an intermittant flickering bat. light. I have gone over and checked the battery, reads 12.5 volts with car off, 13.5 with car running. So the alt. is charging. Whe the light does flicker, it's not at full brightness all the time. Need some help here. Could it be the regulator going? That would be nice, since it's the cheapest and easiest part to change vs. the alt.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member