AT LAST, THERE'S some cheerful news out of Detroit. General Motors, feeling confident after an improvement in sales during October, announced that it had enough financial strength to hang on to Opel, its European subsidiary, rather than sell it to a Russian-Canadian consortium. Chrysler rolled out a new turnaround plan. And, most promising of all, Ford -- the only American carmaker to avoid a federally funded bankruptcy reorganization -- announced a billion-dollar third-quarter profit, boosted by a substantial increase in its market share.