Sorry for the extremely late reply/follow-up.

I have a W123 (1982 300D) and replaced the original speakers with Helix Esprit E4X and E5X speakers. The 4" Ex4's drop in as long as you break off one of the four mounting ears.

If I remember correctly, in order to fit the 5.25" E5X's I had to make new mounting holes on the rear deck, and had to ditch the old original grilles. The Helix grilles are pretty discrete looking.

I also changed the Becker for an Alpine 148BT HU.

I work out of my car and need the bluetooth phone connectivity.

I'm very happy with both the speakers and head unit. A bit of noise picks up from the mic when I am on the phone, but I started out with low expectations.

Hoping to install a JL Audio TW3 series subwoofer in the trunk sometime soon.