Once you acquire the Supra (Details
here) there will be an area on the map to travel to, search this area for the Ruf and tail it for a few meters and you will be given a target to beat the Ruf to.
Once you make it here you will start a showcase event against an attack chopper, beat the chopper and the Jeep is yours once you return it to the garage after finding the barn find (You can fast travel from the map using
).