Well we tried in vain today to access the horn on the 08 M3 with fog lights but I have not the slightest clue how to access it. We got underneath and removed the black panel that drops down, but could not for the life of us figure out how to reach the horn. From underneath you cant even see it! If you look through the grill, you can see where it is and if there was a way to remove the fog lamp Im pretty sure it could be reached.... any ideas? Do you have to remove the entire bumper?

 

I didn't have to remove my bumper.... really I just removed that plastic piece and Viola! there was the horn. It's possible there are changes in later model years, but I don't think there are. If you feel like removing the bumper then you can... should only take you about 15 minutes.

 

Ok, on mine there appears to be two plastic sections. One piece, the larger of the two, is closest to the wheel wells and sort of drops down from the front but is attached by some funky lookin clips to a black piece that looks attached to the bumper area... I didnt remove this front section because i didnt think I would be able to reach it still. Did you remove both sections? Or does anythingI juy tried to explain make any rational sense at all? lol, sory for my lack of technical terms here...

 

The plastic part I removed was the part directly underneath the driver's side fog light. Perhaps removed is a touch harsh.... I displaced it. That same piece continues up into the wheel well. I also removed several of the screw/connectors in the wheel well so that I could bend the entire thing more out of my way to make working on it a bit easier. It's definitely easier if you can remove the front left wheel, but if you can just turn it then it will give you the clearance you need to remove those connectors.

Here's a pic of what I "removed".

 

awesome! Looks like I only removed the back part then and not the front section. We will try that this weekend and see what happens.

 

Actually, that's the front of the car, Mike.
I did the horn mod w/o removing the bumper and found it to be a PITA. Then several months later I was doing an exploratory and CAI install and removed the front bumper. It woulda' been so much easier this way. Like Spots says, it only take about 15mins to remove the bumper cover. To me it would be worth it.
Here's a pic I know I've posted before during the CAI install....

 

Is there a guide that shows how to remove the bumper correctly?