I also found this website that has speaker and stereo mods for the R170 SLK. Like adding tweeters the A-Pillars and also adding I think it’s a sub or two in the passenger footwell. I remember a guy on the German XF forum did something similar and just added full range speakers in the rear.
Thought this might interest some. https://www.dachmodule.de/category-4...edes-SLK-R170/