I was able to get the wiring working on my w123 with a guide from Benzworld. I've used it often when the fog in Ithaca was terrible.
I'm not sure if the wiring between the w123 and w124s is the same. I guess if you were feeling adventurous then go ahead and find out
http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1396820-how-get-rear-foglight-s-working.html