If it works on the right side but not the left it sounds like a receiver module problem. The right side works off a relay and the driver side works off the module. Depending on the year your module is under the dash cover using a 5 pin receiver or above the gas pedal beside the steering...
To all those looking for a keyless entry receiver part # 16254970. GM has discontinued this part. Through luck and research this identical part is available through Kenwoth or Peterbilt as part #p21-1004-2. My cost in Canada was under 80.00. This is the 16 pin receiver located alongside the...