I don't think there should be anything there other than spot welds holding the rear and side panels together. Modification for some reason by a previous owner?
Thanks. It was a rebuild from the ground up, but originally a 2.25 diesel he bought back when we were stationed in the UK. I don’t think these panels are original so it may have some purpose behind the panel. Seems odd to have used a modified side panel as a rebuild part.
It’s a friend’s truck I drive maybe once a month to track wear, look for leaks, and keep it running because he doesn’t right now due to work/life. One of the sides is stainless the other wasn’t and I may look into it for him because it’s the only rusty looking thing on it.