So as I was planning earlier, I upgraded the horn of the 2019 SL Plus to a set of FIAMM Grandes.
Here are a few observations from the install:
- I did not need to take down the entire wheel well - just the part that is immediately below the washer fluid tank (and the bottom shroud as well, just 1 screw) - that forward portion of the wheel well bends down; the opening is not large, but it is usable; also - the SL Plus has the blue accent strip that I found hard to remove - if that can be removed, the access gets a lot better.
- I used 2 L brackets from Ace hardware that I connected together with 2 bolts/nuts to form a T bracket, then bent the bracket into a U shape, so after mounting the horns, they sort of face each other, and the whole contraption is more compact as a result; the upper bolt/nut combo is also used for mounting.
- I was able to fit my 2 horns through the opening, and with some scraped hands and a lot of cursing I was able to tighten the mounting bolt on the same tab the original horn was mounted on; the contraption hangs freely, and it seems there is plenty of space going towards the middle of the car; I used a normal socket wrench and pliers together to tighten the 1 bolt/nut (with washers on both sides) the contraption is hanging on.
- After sealing everything back up, I'll admit the horn could have been louder in a different position (such as the engine compartment), but I am happy I was able to install it in the same spot as the original horn.
- It took me several hours to do this, mostly trying to figure out how to position the horn contraption and how to tighten the mounting bolt.
So the bottom line is the new horns CAN be mounted in the same location as the original with a little bit of elbow grease, and the whole wheel well does not need to be removed. Sorry I have no pictures, but I tried to make the description as detailed as possible.