DETROIT — The purchase of an average-priced new vehicle during the third quarter of 2004 took 20.8 weeks of median family income, before taxes, according to the Auto Affordability Index compiled by Detroit-based Comerica Bank and released December 2.This is 0.3 weeks less affordable than second quarter´s revised 20.5 weeks. Total cost, including financing charges, came to $27,642 in the third quarter, 1.9 percent up from second quarter 2004, and 2.3 percent above year-earlier levels. Median family income rose 0.8 percent from the second quarter, and is up 3.8 percent from a year ago."New vehicle purchases in 2004 are still on track to be at their most affordable level since 1978," said David Littmann, chief economist, at Comerica Bank. "A year earlier, the average vehicle purchase absorbed 21.1 weeks of pre-tax income. Year-to-date the absorption rate is 20.8 weeks, versus 21.0 weeks for the comparable three-quarter average in 2003."