The battery keeps on running flat on my NA6, (no I'm not leaving my lights on)

HISTORY: A couple of weeks back, while driving to work, the radio started cutting out, the wipers were moving real slow and the dash lights started dimming. The charge & check Engine lights came on also. My first thought was a short in the wiring under the dash, but when I went to take the car out at lunch, it wouldn't start. So I got it charged from a plug-in charger, took it to the mechanics and he tested the battery & Alternator, he said the Alt is fine but I need a new battery.

New battery installed, the car ran fine for about a week and a half & then the exact same thing happened again, and once again tonight.

Now I am thinking that with Aftermarket stereo, EDFC and Head/Tail/Dashlights running I am drawing too much power from the battery. Is that Possible or would it take a lot more than that to drain the battery? are there any other likley causes? I don't know if it is relevant but everytime it has happened the temp has been around 40-50°f. Anyway, I will try to get it into the mechanincs sometime this week.

Thanks,