You're looking for a number (last 5-6 digits of the VIN). As an anti-theft thing, Mopar put that same number on the radiator core support (not the radiator). It is on the passenger side, back side & under the "lip" of the core support. Also, that same number will be under the trunk weather stripping, driver's side, on the body of the car itself, just behind the lower, driver's side corner of the window (& back a little bit near/under the trunk weather stripping). If it is originally a Road Runner, then the VIN tag on the dash, the fender tag, the core support & the trunk area will all have the same number.