For the third consecutive quarter, Comerica Bank's Auto Affordability Index has fallen. In the first three months of 1996, it takes 28.8 weeks of median family income to purchase an average-priced vehicle, compared with 30 weeks' income during the fourth quarter of 1995. Lower interest rates, rebates and consumer optimism about future employment all contribute, says Comerica Senior Economist David L. Littmann. "The average financing rate declined by more than one-half percentage point, and the average consumer outlay, after rebate, came down $233," Mr. Littmann says. "Low unemployment and good employment gains lifted median family income by more than $500 during the