Just install the backported spamassassin. Add this to /etc/apt/sources.list.d/debian-wheezy-backports.list:
deb http://ftp.nl.debian.org/debian/ wheezy-backports main contrib non-free
deb-src http://ftp.nl.debian.org/debian/ wheezy-backports main contrib non-free
and run:
$ apt-get install -t wheezy-backports spamassassin
