Unbeknownst to most Red Hot Chili Peppers fans, bassist Flea voices ''The Wild Thornberrys''' manic Donnie on Nickelodeon. Here, on the Paramount lot, he lays down vocals for the June 13 Rugbabies-meet-Thornberrys film ''Rugrats Go Wild.'' Bruce Willis may voice the Rugs' pooch Spike, but Flea's ''yuckatychuckatyluckaty'' steals the show. ''There's definitely a part of me that is completely him,'' says the father of one. ''It's also the most bizarre, strangest job I've ever had.'' This coming from a man who's sheathed his privates in a gym sock.