GM Gives Car Buyers 'Locked-In' Rates  DETROIT (Reuters) - General Motors Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=GM.N target=/stocks/quickinfo/fullquote"&gt;GM.N&lt;/A&gt; will offer  car buyers nervous about rising interest rates a chance to  lock-in today's low rates in a bid to boost its sluggish U.S.  sales, car dealers briefed about GM's new sales incentive plan  said on Tuesday.