That is normal for US 123s.

The socket is for the rear high intensity fog lamp that is actuated when pulling the light switch out to the second stop. If you pull the knob off you will notice a light socket there, that is supposed to light up with the knob pulled out to the second stop and makes the hub of the switch knob glow orange.

The Feds don't like that feature or at least they did not back in the 80s so the US models were not equipped.

I think several members have added extra brake/tail sockets salvaged from a parts car and wired in parallel to the other brake/tail bulb. On a 123 that is a good thing given there is only one brake light per side and no CHMSL.

Or you can get hold of the Euro wiring diagram and hook up the rear fog feature. I believe the US light switch has the necessary contact and the indicator bulb socket in the knob stem is active. It uses the same type bulb as the turn signal. This goes on the road side (left for US), the other side remains blank.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech