The article is called “Fodor’s Guide to Mental Representation: The Intelligent Auntie’s Vade-Mecum”, published in Mind, (New Series, Vol. 94, No. 373, Jan., 1985, pp. 76-100) and reprinted in Readings in Philosophy and Cognitive Science (ISBN 0262071533).
The whole article doesn’t seem to be available online, but the first page does seem to appear on JSTOR.
The opening paragraph is wonderful:
It rained for week and we were all so tired of ontology, but there didn’t seem to be much else to do. Some of the children started to sulk and pull the cat’s tail. It was going to be an awful afternoon until Uncle Wilfred thought of Mental Representations (which was a game we hadn’t played for years) and everyone got very excited and we jumped up and down and waved our hands and all talked at once and had a perfectly lovely romp. But Auntie said she couldn’t stand the noise and there would be tears before bedtime if we didn’t please calm down.
Link to first page of ‘Fodor’s Guide to Mental Representation’.