I just finished replacing the cable on my 84 rabbit TD. It may be a different setup for a mk2. But the only way it wanted to work for me was to pop out the rubber gasket in the firewall (new cable came with one on it) and pull it out from the engine bay, then push the new one through from the engine side. it comes through just above and to the right of the the pedal assembly inside, and can be bent around to the proper position from there. getting the end of the wire through the eye at the backside of the pump can be tricky though, long needle nose pliers and a screwdriver for a pry bar and some fiddling around is about as detailed as i can explain it.
-James.