Actually there was an optional equipment fuel tank for SIII 109s that allowed fitting of a rear PTO. It was a half size tank that allowed passage of the propshaft.
edit: I originally said station wagon, but old timer's is getting me. It's for 109' non-station wagons that had a rear fuel tank.
I picked up my rear PTO from ebay, they show up now and then.
There are actually two major components to the rear PTO. The center PTO, which is fitted to the transfer case, and the rear PTO in the photos. The propshaft of course is needed, but that could be sourced locally in a pnich.
If you get one, make sure the center PTO is not for a 1 Ton as they are not interchangeable between the 1 Ton transfer case and normal Series transfer case.
If I can remember, I'll look at the difference this weekend (I have one of each) and let you know how to tell them apart.
I do know it's a different sleeve gear, bearing and input shaft.