Does the unit you are looking at look anything like the items in these pictures?
If so you can find a little more info on the hook-up in the 560SEL Accomplishments
thread, buried somehwere around page 4 or 5. The hook-up for a 500 ought to be similar.
Hope this helps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.