The Jetta is equipped with a US made and supported Alfa Pro rally (http://rally.cc/) computer to measure time, speed, and distance.
The Alfa uses a switch to ground to trigger the pulse counter to measure distance- for cars with a VSS, it will interface directly, but on the Jetta, we used the included Hall effect probe and magnets.
The magnets were glued inside the rear left rotor, with the probe mounted off an ear from the rear stub axle fixing bolt. This worked great until the rotor finally succombed 5 seasons of use and cracked. In the rush to replace it, something went wrong with the magnet mounting leaving the unit unable to measure distance.
A Garmin Zumo GPS measuring speed and distance has been the extended temporary workaround for the past couple of seasons, but definitely wasn't optimized for rally use.
With the Jetta starting duty as a rental the rally computer made the list of "must fixes".
An email to Mike Friedman later, and I had a new probe and magnet kit sent out. We are using the same basic layout for the probe, but with a removable ring mounted to the inside of the disc.
This only requires that the rotor be drilled and tapped to mount the ring. The holes are easily layed out with a caliper, then drilled and tapped by hand from the front side. The magnet mounting disk is reusable when the time to replace the rotors comes around again, which should prevent a repeat of the issues encountered last time.
Serviceable, durable, and done.