E300D Battery Going Dead After a Couple Days

[DAR2KMR2]

I have a 1996 E300D with an older battery so when I heard the starter begin to struggle with starting the car I ran down to Sears and had the battery checked out. They said the battery was bad and sold me a new one. That was back in May. The car was driven daily until a month ago ... then we discovered the car had to be driven daily or the battery would go dead. Ran the car back down to Sears they tested the electrical system and said the Alternator was good but the battery was bad and gave me a new one for free. That was great but it didn't solve the problem, the battery still goes dead after a day or two of not driving. I checked the easy stuff like making sure the trunk and glove box lights go out and I couldn't find any obvious problems with the lighting functions. Any ideas where I should start investigating? Any tricks to finding the culpret circuit? Thanks for any ideas/help