Try some hd 3/8 inch foam tube on body side to squash the door seals tighter. The inverted u shaped latch on bottom can be shimmied up but best way is to put a thin piece of delrin sheet on the little ramp that guides door up and into place
. I want delrin bushings in my hinges as lubes just dont last

 

In my case (G20 with opposing doors) it was the striker bolt from the smaller 1/3 door. The 2/3 is supported by this bolt with it's weight. I used a PVC tube with ~ 1cm diameter cut to 2cm and cut it longitudinally to squeeze it over the bolt where the door snapper grabs in. The door closing sound is already way more solid and the rattling and squeaking are gone.

 

I know this is an old thread but this may help someone.

I had a similar squeak somewhere in the rear of my 2011 cargo. I couldn't find it on my own so I had someone drive it while I rode in back. I was able to pinpoint it to the lower latch on the rear barn door. I was able to recreate it by lifting on the door while it was closed.

I sprayed some white lithium grease into the mechanism while working the handle and it's been quiet ever since. I like the lithium grease for hinges, latches and other moving parts. It lasts longer than penetrating oils and dries with a waxy paste in the crevices that creates a barrier for dirt and dust. The lithium doesn't penetrate nearly as well. When I need it to penetrate into a tight mechanism I'll start with some WD40 until the squeak stops, follow up with the lithium then a bit more WD while I move the mechanism to work it all in. The WD will thin the lithium and carry it in. The lithium will remain after the WD dries and goes away.