Buy OEM 2003 GENERAL MOTORS Part # 25863021 (25789211). Door Latch Assembly (Front). Silverado, Sierra; w/o LT, SLT; w/o All Terrain Pkg; Left. W/power locks, w/o SLT pkg, w/keyless entry. W

Lock. Latch.
Mechanism to secure door to chassis. Actuator. Assembly - Side. Door Assembly.

Door Latch AssemblyDoor Lock Actuator Motor

Fits Sierra 1500, Sierra 2500 HD, Sierra 3500 HD, Silverado

Silverado, Sierra; w/o LT, SLT; w/o All Terrain Pkg; Left. W/power locks, w/o SLT pkg, w/keyless entry. W/power locks, w/o SLT pkg, w/o keyless entry.
Door Lock Actuator - Repair or Replace
Your 2003 may need a new door lock actuator if your keyless entry doesn't work or your lock button doesn't lock the doors.