We had a couple of guesses, but I kinda figured this one would stump some of you.
According to the credits, the manure dumped on the convertible in Harper Valley PTA was "courtesy of Seattle Slew".
Wikipedia states: Seattle Slew (February 15, 1974 – May 7, 2002) was an American thoroughbred race horse who won the Triple Crown of Thoroughbred Racing in 1977, the tenth of eleven horses to accomplish the feat. He remains the only horse to win the Triple Crown while undefeated. In the Blood-Horse magazine List of Top 100 Racehorses of the 20th Century, Seattle Slew is ranked ninth.
Oddly, neither Wikipedia, nor Seattle Slew's fan site (so far as I've been able to find) mentions his contribution to the film industry.
New question on Saturday!