As a Ford Motor Co. executive in the far corners of the world, L. Lindsey Halstead has seen his share of political and economic turmoil. In topsy-turvy Brazil, the Ford veteran recalls spending more time "riding a financial bicycle" than building cars. These days, in the wake of the violent storms sweeping through Europe's currency exchanges and political corridors, Halstead, now chairman of Ford of Europe Inc., admits he may have a similar problem: "We've all been shaken." It wasn't supposed to be like this. Since the mid-1980s, Europe's companies have been thoroughly overhauling themselves, prepping for the open competition that will begin on Jan. 1, 1993. Along with lowered trade barriers, executives had been expecting a Europe of low inflation and strong growth. The biggest payoff of all was to be a single currency, designed for one large market.