if, in keeping with the general MB philosophy of making parts as universal as possible between model lines, the system is the same as the W201, and i have little reason to believe that it would be different, the feet of the triangle will slip into two holes at the bottom (front edge) of the trunk lid and the plastic bracket that retains the top of the triangle will slip into the holes at the top (back end) of the lid and slide to one side to lock into place.
if your W124 trunk lid has a lining,as in the 500E/E500, you will have to remove the lining to see the mounting. in order to use the triagle, you will have to either cut out the moulded triangular section of the lining or omit the lining altogether.
hope this helps.
'94 W124.036 249/040 leder; 8.25x17 EvoIIs
'93 W124.036 199/040 leder; 8.25x17 EvoIIs, up in flames...LITERALLY!
'93 W124.036 481/040 leder; euro delivery; 8.25x17 EvoIIs
'88 R107.048 441/409 leder; Euro lights
'87 W201.034 199/040 leder; Euro lights; EvoII brakes; 8x16 EvoIs - soon: 500E rear brakes
'70 R113.044 050/526; factory alloys; Euro lights