My Battery is Drained after 3 Days!!

My '96 Deville has been giving me a headache lately. I woke up a few weeks ago with a dead battery. I gave it a slow charge for 18 hours and it started up. I let the car sit for a week and it was dead again, this time it wouldn't take a charge. The battery was only a few months old so I returned it and got a new one. Now this one is dead already. I charged it back up and tried testing the voltage of the battery while pulling fuses under the hood and saw no change. I know there's another fuse box in the trunk I can try but is there anything on these Devilles that are notorious for draining batteries? A common problem anyone else encountered? Any advice would be greatly appreciated. My last hope is to pull the fuses in the trunk one by one but it's like -5 degrees so I am trying to avoid spending any amount of time outside. The amps are no longer connected, nor is the alarm. Just trying to rule out any aftermarket equiptment.

At -5 outside, I suggest to take to auto-electric shop or dealer. Draws can be very time consuming to correct and weather is against you. Local Vo-tech can be a low cost alternative since they don't normally charge labor.

I took the car to a shop. After 3 days they gave me my car back and said they couldn't find the draw. I don't think they spent more than a few hours on it.

Get a Battery Tender , they only top up battery, not like a trickle charger that can fry a battery, they run about $39.00 , I have one on corvette in garage for winter from October through to April.  Look on internet for Battery Tender.
 
Otherwise you will spend days and days to figure out what is draining , or what is not shutting down, when you turn off key