Ford Down, Nissan Up in Sept. U.S. Sales  DETROIT (Reuters) - Ford Motor Co. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=F.N target=/stocks/quickinfo/fullquote"&gt;F.N&lt;/A&gt; posted its fourth  consecutive month of weaker U.S. sales with a 7 percent drop in  September results on Friday, and the automaker doubled its  incentives on some models to kick-start sales this month.