Impala Trunk lock not opening

Hi Chevy Guys and Gals
We've had classic Chevys for 30 years; never a problem with the locks, except sometimes getting a little loose.
'64 Impala The trunk lid used to open with a quarter clockwise turn. Recently there was a little resistance before opening. Now I can't open the trunk!
Now the key goes a half-turn before stopping, but not opening. The lock assembly is not loose. The trunk lid does not move; it's solidly on there.I tried using 3:1 oil wd-40; no go.
What should I (or my mechanic or locksmith) do?
Is it best to get a new lock (what manufacturer?) or used?

what's good for lock maintenence? My dad used powdered graphite, but I hear 3:1 Oil is better.
Thanks
Rick G :banghead:

Sounds like the lock assy not the key cylinder, I would try and get in the trunk thru the back seat area and get it open, then you will be able to see what the issue is.

i had the same problem,i had a 64 impale and the lever from lock to tumbler,twisted and would not open or engage lock,had to get in thru back seat to open then preform fix on lever. i thought i could straighten it, but it make it weaker,so ended up buying new one. hope that helps

Hey Guys
Thanks for the advice about going thru the back seat area into the the trunk.
I removed the seats found some rust underneath.
It's going to be pretty difficult to get to the lock as my Impala has (4) 3 inch welded braces forming a 19 inch wide gap at the bottom of the trunk top and 6" at the top.
Trying the easiest possible way first I notice in looking at the lock area the was about a 1.5" "flange" metal on the side of the lock. I thought it would be like a lever to release the lock. I put a large block on the trunk top and with a long metal rod attempted to leverage up to open it. I was able to put some force on it but no go.
So if I get in there what should I bring with me, tools etc. and what should I do?
thanks for your help, Rick G.

Should be a u shaped clip holding the cylinder into the lid, if so pry that out have someone outside to remove the cylinder, that will allow you to turn the lock manually.- Pliers, medium screw driver, small hammer.

TRUNK LID OPENED! YEA!, CYLINDER ASSEMBLY NOT COMING OUT. AHH
Thanks for the advice so far. Once I got into the trunk thru the back seat area I was lucky to get the lid open easily by seeing a .25" slot beneath the cylinder assembly then turning that with a screwdriver, and having my brother outside turn the key.
I was easily able to remove some heavy metal latch? part by removing 3 bolts.
We noticed the tailpiece which turns with the key to open the lock appears to be broken.But after removing the 2 screws for the cylinder assembly underneath the trunk lid, my brother who is a mechanical engineer and works on his cars was unable to get out the clip even using the tools reccomended.
Any other tools, tricks, ideas we might try?
Thanks, Rick G.

Probably a little rusted/ stuck, soak some PB blaster or such overnite, you must bend the flat lip of the clip towards the lock and pull it out at the same time, hard to describe from here.