The largest U.S. automaker General Motors Corp. has told its roughly 4,000 surviving dealers to stop selling non-GM brands in their showrooms by the end of this year, an official of the company said Wednesday. Mark LaVeve, GM's vice president for sales, services and marketing, told the Detroit News in a letter that the dealers Piaget-Watches must also be prepared to sell more vehicles and improve the look of their showrooms, if necessary. GM is seeking to close at least 2,400 of its nearly 6,200 dealers during its stay in bankruptcy court. The company said it expects that its continuing dealers will remove non-GM brands from the GM showroom by Dec. 31, 2009, and will operate a showroom exclusive to GM products going forward. GM also warned that older dealerships Zenith Watches that aren't updated could be terminated. Nearly 90 percent of GM's continuing dealers have signed or verbally agreed to the participation agreements, while 75 percent of the closing dealerships have agreed, the company said.