Buy a DieHard Gold if you get a new battery. My car still starts at 30 below even when it's been sitting for weeks. The warranty is pretty good, too.
Hook up an ammeter at the battery to see if you actually have a drain. The CAA guy said it was a 2.9 amp drain so I'm assuming he didn't guess at it. With the ammeter hooked up, start pulling fuses from the most likely suspects until the drain goes away. That will help you pinpoint your problem. If you don't have a drain, replace the battery.
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351W 89 Mustang GT Convertible