Thought I'd share my experience installing new speakers in my 1988 300E. After reading through all the posts I could find here, there seemed to be some debate as to whether or not a 5.25" speaker will fit in the stock dash locations in the W124. Since I had found a great deal on a set of MB Quart Referenz QM130.03kx 5.25" coaxials, I thought I'd attempt to install them. A total stealth install was very important to me. I did not want any indications at all that this car has anything but a stock stereo in it.

First of all, in order to do this, you need a Dremel tool with cutting and grinding bits and the willingness to use it. A LOT of material is removed. If aggressively cutting the dash of your Mercedes makes you nervous, do not attempt this install. The hole in the aluminum dash was selectively enlarged by about 1/4", the vinyl padding was trimmed away from the hole and the steel seam on the firewall was trimmed by about 1/4" or so (the steel gets hot and the paint starts to smoke, make sure you have water and a fire extinguisher handy). The ledge that the grill rests on also needs to be relieved to make room for the speaker rim. The lack of depth on the drivers side requires a 3" or so hole (the diameter of the magnet) to be cut in the top of the air tube feeding the vent. If you measure and cut very carefully, the speaker magnet pushes in to the hole. A few rubber bands around the speaker magnet helps seal it. I checked the airflow after installation and did not notice a decrease.

These are coax speakers with a tweeter built into a bridge over the the larger speaker, so they are taller than a single speaker would be. This makes it difficult for the grill to seat completely. The bridge needed to have the corners rounded in order to clear the grill, and the perimeter of the speaker housing also needed to be ground selectively to make room for the grill. This is a fit, test, grind lightly process that was repeated many times. If grinding on high quality speakers makes you nervous, don't attempt this install.

In the end, it all worked out and the speakers sound awesome. I know some people find MB Quart speakers bright, but I find them to have incredible detail. The tweeters sound great reflecting off the windshield. They are powered by a Arc Audio 70wpc amp running through a Arc EQ.

If you are considering doing this, make sure you are comfortable with cutting your dash. Once you start, you are committed. Work very slowly and carefully. I ended up spending about 12 hours just getting the two speakers in. I'd recommend doing each side on a different day, ideally over a few days, because if you get tired, and the Dremel slips, you could damage your dash in a microsecond.

Hope this helps anybody considering this install. A couple other pointers: I found it easy to run the RCAs and the power wires to the trunk through the plastic channels. Just convince yourself your car needs a good cleaning and remove both front seats and the rear bench and backrest. After pulling up the sills, the carpet lifts easily and running the wires is a cinch. I ran 4 gauge power down the passenger side and the RCAs and amp turn on signal wire down the left side.