Battery draining on 98 Ford mustang.

If it is in fact true that your battery loses electrical capacity overnight as the vehicle sets, a parasitic draw drain test will be required. See this article for some details: https://repairpal.com/parasitic-drain-diagnosis Beyond that, your starting and charging system should be tested. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops