93 Mustang need help

I have a 93 Mustang that is not charging properly. I have had the battery tested and charged. I have replaced the alternator. When I replaced the alternator I tested the battery with a volt meter and it was at 12.6. When I start the car the volts drop down below 9.0. Can anyone give me any other options to check.:runaround:

At idle, the voltage should increase to above 13V to keep the battery charged. I'd look for a grounding issue.

Thanks is that a ground for the battery or alternator.