[brcidd]

The same two wires are used in either lock or unlock, just reversed polarity- so that is not the issue.

The thing I see is GM's lovely sluggish actuators- I have replaced many in pick-up trucks, and vans etc-- seems they get very slow and weak over time and I generally replace them with aftermarket actuators I buy online. They seems very strong and able to do the job.

So when you open up the door- measure the voltage at the actuator and see if it is fine-12 volts positive and negative-- if actuator is sluggish, you can try to lube the rod area- or check for obstructions, but most likely you have a weak actuator that needs to be replaced. I recommend aftermarket, only because they are much less expensive, and you should not get into the same problem as you would from a boneyard. Plus I doubt a boneyard would sell just the actuator--maybe a really good one would..