Originally Posted by RedHorizon
Looking at the pictures it's tough to tell...but are you sure it's veneer? .
It looks like a typical wooden cabin construction. Sometimes full width planks were found(old growth forest days), but as often, the back were built up of two or several planks. They were usually joined with a tongue in groove joint. As wood contracts and expands, compression sets in at the joint, and you get an open seam. Once varnish fails(that's your seal against moisture), the joint quickly deteriorates.
Usually(and it may be the case here), a new spline is added to fill the crack. If it's irregular, it may be routed oversized a bit. You may have to do that for a tight joint or you maybe able to just pull the old spline out. Clean the pocket, and fit new.
It's an easy process if you have some experience. Make the spline a bit fat, and sand or hand plane a slight bevel-epoxy-tap in. Once dry, carefully plane it near flush-sand.
I've had to do that in several areas of my wooden cabin. Over 10 years ago, I 'wooded' the back(your's needs that done). If you look closely, you can see the one joint just above the bridgedeck. I had to add a 1/4"x1/4" spline which is epoxied in.
Then if you build the varnish back up(8-10 coats), add maintenance coat once a year(in New England), you're cabin back will be good for 10 to 15 years.