General Motors' August vehicle sales fell 5% to 435,597 units from 443,089 for the same period in 1999. Domestic-built car sales slipped 1.7% to 232,007 from 235,390 for the 1999 period. Domestic-built light truck sales dropped 4.4% to 1,656,718 from 1,587,145 for August 1999. Total truck sales fell 5.5% to 198,542 from 201,785 last year. Total light truck sales dropped 5.5% to 196,385 from 200,057 a year ago. Total light vehicle sales slipped 5.4% to 433,440 from 441,361 in 1999.