Difficult to say about fitting and wiring because the W124 is from an era when such exotica as radios and aerials were not standard equipment.
But there are two factors to consider when mounting the aerial: the backwards tilt that you mentioned, and also the steep lateral angle at which it cuts the rear panel. (Much more than the 35° that the 2040 mentions!)
My 1990 230E had a typical after-market lash-up fitted. The aerial is from Harada, the wiring was passed through the easiest route from back to front (i.e., stuffed under the carpet and not run through the ducts with all the factory-fitted wires), and the hole in the panel was definitely not cut in the factory!