SAN FRANCISCO -- The code for San Francisco 49ers to succeed under Jim Harbaugh has not been difficult to crack.It begins with the defense, a unit that is dominating despite playing without All-Pro linebackers Aldon Smith and Patrick Willis.Sure, the 49ers have been pretty spectacular on offense at times. But this team lives and dies with its defense. And if the 49ers stick around as serious contenders in the NFC, it will be because of the defense.