going back in Request forum asked for the live version of IILWMC with Freddie's vocal on it, but as I know although I ain't a Queen lives expert, it was only one occasion. He usually sung "Oh yeah" after Roger's "I'm in love with my car" line and no more.
From Martin's (Mr Scully) www.queenconcerts.com
Glasgow, 1.12.1979 "Roger completely forgot the words to 'I'm In Love With My Car' and after an "instrumental part" Freddie realized Roger's blackout and had to sing a couple of lines to help him."
But maybe there's version with more singing lines by Freddie or this one in best quality, I don't know. http://rapidshare.de/files/1510067/I_m_In_Love_With_My_Car.mp3.html