Debian feels very non-standard here again (as usual). For instance, in Debian Jessie, the physical ar program usually shows up as
/usr/bin/i486-linux-gnu-ar resp. /usr/bin/i586-linux-gnu-ar
resp. /usr/bin/x86_64-linux-gnu-ar.
Though I have no Debian here at my disposal, I assume that /usr/bin/ar is merely a symbolic link. (Please note that this is just an assumption - it would require me direct access to a live system to verify on location.)
So maybe the following line will help you out of your misery, provided your aptitude works:
$ sudo apt-get install binutils
In the negative case (i. e. if it does NOT work), download e. g. binutils_x.yy-p_i386.deb (x.yy is version number (e. g. 2.25), p is patch level) from a Debian FTP server and install it using dpkg -i (must be superuser).