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.