In addition to being the name of a plastic toy from the 60's, the term rat fink was once used specifically to mean a narc or stool pigeon. Today, it's used generally to mean a despicable person. This is part of a complete episode.
My dad, born in 1942, uses rat fink as a swear word. He has always used it as an exclamation when frustrated, angry or generally ruffled. Say he was fixing the plumbing, he could be heard muttering "Ratfinks!" But because my dad is deaf, we could hear his "swear" in the next room. I thought this was really interesting given the title of this show -- Hell's Bell's.
Rat Fink was also a Hot Rod related comic character in the 50s and 60s - reference the wikipedia article or the image results of a Google search on Rat Fink. I can see how you could take the concepts of Rat Fink and hot rods racing around and apply the term ratfinking to a pair of dogs zooming around looking all crazy.