Meanwhile, back on the original topic:
A word of warning. Mandy and I once had some old margarita mix which we had kept refridgerated. It was a few weeks old when we decided to use it to make some more margaritas. We figured it wouldn't have gone bad. Wrongo! We were both sick with food poisoning for the next 2 days (and when I say sick, I mean SICK).
I know your mix is probably not that old, but given my experience, I would say when in doubt, toss it out.