Interesting problem!! And a very clever solution? A U-turn socket!...
Yes, blocking the holes should produce more and better sound. You might try some absorbent material, but simply adhering a thin piece of wood (that doesn't interfere with the grille) over the holes will work better. Use an adhesive that can be pried loose later if you need to go back in. I'd patch that gap at the bottom as well. You might also consider paintng the wood black, so it doesn't show through the grille.
Stereo wasn't exactly a super common thing in cars back then. I am an electrical engineer with a serious audio hobby. I have been building speakers ever since the late 60's (Yikes, how did I get that old?!). I have been pondering how to get good sound in my E9 without going too far off the original setup.
I see you are in Bilbao. Always wanted to go there. In October, we will be traveling from California to Barcelona and along the coast to Lisbon (my closest friend moved there from California last summer). I have been many places in Europe, but never to Espania. I live about 60 km north of San Francisco, in the California wine country.
