Communication towers those blinking, guywired harbingers of car phones have sprouted at the rate of 5000 a year nationwide for the past decade. Industry watchers say another 100-thousand of the structures could go up by 2010. While digital phones mean improved service, the towers themselves -- often topped by flashing strobelights arewidely abhorred as a blight on the landscape. In the Brainerd lakes area, county officials are trying to bring them under control.