'03 3.4 Impala: Trunk lock won't unlock w/key

Tried spraying WD-40 into front and backside of lock. Also tried grabbing the rear piece inside with pliers and turning it at the same time as the key on the outside. Can't get it to turn using the key (it turns clockwise to unlock). Any other ideas? Electrical release inside works, as does the pull cord inside the trunk. Just not the lock.

Do you have a spare key to try in the trunk? Could be a key that is getting worn out. If the spare doesn't work it could also be a bad lock cylinder.

They've got two keys. I didn't try the other one because the claim was that it never worked from day ONE and they never knew it because they always opened the trunk using the remote or the button on the dash.

I'll try the 2nd key, but it's looking like a bad cylinder. If I unbolt the latch, remove and replace it, will it still use the same key? The key doesn't appear to go into the latch, only the part on the face of the trunk ... buy maybe I've got it all wrong.