I have never heard of it being a common problem. My 2000 GT can sit for months without any battery drain.
I came upon this thread by accident. For more responses you need to post it in the 96-04 forum under Modular Mustangs. We have a few years to go before our cars become classics.
ProCharger P-1SC, 9 psi, STD 396/383; Uncorrected 388/375; SAE 383/370.
USAF 1969-1973,1980-1996: Vietnam veteran. Aircraft maintenance. R & D, ICBM Operations.
Also own: 1997 Harley FXDWG, 1998 F-150, and 2002 Corvette LS1