04 Sierra Crew Cab - power door lock/unlock not working!

[carcrazysincebirth]

04 Sierra Crew Cab - driver's door power lock/unlock is no longer working! It had failed about a week ago, but then started working fine on its own until this morning. I am wondering if there is a common problem with these, such as an actuator. If it sounds like an actuator, could somebody give me the steps after removing the door panel as to how I can fix it. If this problem sounds like something else, could you please let me know. The left rear door lock/unlock is starting to make a squealing noise - I don't know if this is related to the other problem. Thanks