I'm having some trouble with my 2005 Hyundai XG350L's gas mileage, any suggestions?

I was going to get this in place of my stock horn

Also, my air conditioner will only run if it is put on full blast, any reason for that?

As for my horn, I was planning on buying a train horn, would you have any suggestions on where I could get that installed or a good placement of the horns and aircompressor?

Okay that's what I tought it would be. Also, when I roll my windown down about half way and I let go of the button the windw "skipps" or "hiccups" downward, any reason for that?

Gotcha, I also found a gas station that uses 100% gasoline instead of the usual 90%/10% the other stations use, is the 100% going to give me better preformance for my money or should I stick with the 10% ethonal brands?

Would you recommend the cheapest ethonal fuel or the slightly most expensive 100% gasoline fuel?

I also have a few dents in my cars body as well as a prett noticeable gash on the front bumper where I hit a wall, any ideas on how to get those fixed or any products I could use? The gash on the fron also affects the chrome trim on the front left side, how do I get that fixed?

My fog lights were burnt out so I got them replaced, I went to AAP and got the little light bulb for the shifter selector, so those two things are fixed and marked off of my list, I found another problem, I am missing a piece from under my hood, I'll attach a picture of it so you know what I'm talking about

More specifically here.

Still there? Haha, haven't gotten a reply from you in a while

Well crap. Could that be the reason my horn isn't working correctly? Would I be able to get that piece at any auto store or would I have to get a specific part at a Hyundai dealer?

Could I just take the other horn housing out and put in into a bag of rice or something to soak up the moisture or would that not address/ make worse of the problem? And does the other horn housing just screw back on or is there a different way to get it on, there aren't any instructions in the owners booklet for this problem.

For my bumpers chrome trim I'm just going to find a piece off of an old car at my local Pull-A-Part so do any of the Hyundai models have similar or exact parts as my 2005 Hyundai Xg350 that I can get?

And does the front bumper's chrome trim piece come off or is it attached to the bumper?