I recall "fink" and "rat fink" being used regularly in Mad Magazine articles during the '60s. At about the same time, well-known car customizer named Ed "Big Daddy" Roth, created a weird hot-rodding cartoon character called "Rat Fink". Since both Mad and the custom car magazines in which "Big Daddy" Roth's creations appeared were geared toward the male teenager market, use of "fink" or "rat fink" was faddishly popular for a few years among that group. Other than that brief bump in popularity, I don't think "fink" has ever been all that commonly used.