As you know while the speakers in the 123 are a standard size: 4" or 10cm, they are cut away on 2 corners which makes fitting any aftermarket speaker a nightmare.
Here's what I did in the front:
I purchased some lower-end coaxial Pioneer speakers with only 4 tabs and screw holes around the perimeter. After removing the MB speaker grille and speaker, I used a utility knife to CAREFULLY slit just under the "lip" near where the cut aways are on the MB speakers. Make the wire connection, Start from the rear (farthest away from you), work the speaker into the hole and slide the speaker's tabs into the slots you made with your utility knife. Line up the "rear" holes, sink those 2 screws, and you're done.
All in all, a pretty tight fit. I'm not experiencing any rattling of any kind and with the MB grilles back in place, nobody's the wiser!
I'm going to tackle the rears next!