For broken systems I used the script below a few years ago. I you can get '/usr/bin/ar' (from binutils) to work, the rest should be not big problem (although it's not the best written piece of code…) as also the other programs should hopefully be there on standard systems.
DISCLAIMER: you should always check the extracted data.tar manually before running the script, as ist may overwrite important files if you are unlucky.
# Try to install given package name without apt or similar helpers.
# It's a kind of 'quick and dirty' but usefull if system broke down.
echo;echo -=] Please start this script in a directory with rights to write
echo -=] with the path of the .deb file which should be installed as first
echo -=] and the name \(only the name without \'.deb\'\) of the .deb file as
echo -=] second argument.
echo -=] Sample:
echo -=] \'$0 /cdrom/pool/main/a/apt apt_0.5.4_i386\'
echo; echo -=] First argument has been: \'$1\'
echo -=] Second argument has been: \'$2\'
echo -=] Trying to install \'$1/$2.deb\'
echo; echo -=] Creating directory \'$TMPPATH\'
mkdir $TMPPATH -p
echo -=] Copying \'$1/$2.deb\' to \'$TMPPATH\'
cp $1/$2.deb $TMPPATH
echo -=] Unpacking from Debian archive \'$TMPPATH/$2.deb\'
ar xv $TMPPATH/$2.deb data.tar.gz
echo -=] Unpacking from gzip archive \'./data.tar.gz\'
gunzip ./data.tar.gz -v
echo -=] Moving \'./data.tar\' to directory \'/\'
mv ./data.tar /$2.tar
echo -=] Entering directory \'/\'
echo -=] Unpacking tar archive \'/$2.tar\'
tar -xvf /$2.tar
echo; echo -=] Removing \'/$2.tar\'
echo -=] All done!