Who do I see to fix my back door lock ony Sedona van?

I have a 2001 Kia Sedona that was converted into a "camping" van. The back door lock worked when I bought it a few weeks ago, seemed to be completely locked and unlockable for two days, then worked fine while loading it up last Thursday, and showing it off on Friday to my camping companion. Friday night it did not unlock. I texted with the person from whom I'd bought the van, he said he'd never successfully locked the back door of the van at all. I put a squirt of Ed 40 into the lock that isn't the problem. My friend thought there was a latch he tried to access with a screwdriver but he was unable to get to it properly. I got such a good deal on this sweet little "camper.," I am willing to put a bit of money innto it to make my little voyages more comfortable.

So, do I go to a lock smith, a mechanic, an auto body shop or a Kia dealer service center for this sort of thing? I've never had a car have a lock problem other than one being iced up, and this is not that problem.

So who fixes lock problems?