Probably an alternator but you need a hands-on inspection to be sure. If you need to find a shop for this then use this link to find a good one near you.
https://repairpal.com/repair-shops
Alex
RepairPal Certified shops can diagnose and repair your 2003 Mercury Sable. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.