Diagnostic for battery drain on 1993 Miata

[kgmccafferty]

Looking for recommendation for how to diagnose potential electric issue with my 1993 Miata.   I must use a battery tender, and it must be on, to start my car.   Whether it sits for a few days or weeks.  Tests of the battery by AAA and local garage say my battery is good, my alternator is good.    But I can't be sure my car will start if I don't plug in the tender; which is not convenient if I want to take a cruise or overnight trip.    I have been told it could be a problem with the ground wire but I do not know how to check this nor have the tools to investigate.   Any one out there with similar issue or a recommendation on a person or garage that may be able to help figure this out?   Thank you.