Poking around my 96 300tdi, I came across a socket just below the drop down panel for the fuses under the steering wheel. Looks a bit like a TV scart socket. The engine doesn't have EDC and my 95 300tdi doesn't have the socket. Looks like a diagnostic socket but without EDC, there's nothing to diagnose.
Any idea what the hell it's for

By the way, I found out yesterday, if you clean out your K&N air filter with petrol, it's a good idea to dry it out carefully before you put it back. I'm used to engines that don't start. Ones that won't stop are scary

CD multi changer i expect ................. like most of em now long since removed

cd changer got the same in mine

Your right driver it is a diagnostic socket. Must be only used for petrol discos and later Tdi's then, although if u have ABS it would be used for any problems with that system also.