I just removed the horrible stock horn. Replaced with Fiamm Freeway Blaster notes E and F. Easy install. Wiring harness just plugged right in.
Sounds 1000 times better. More like an 80's GM horn. Very loud. Best upgrade for $30 at pepboys for your maxima.

Also I am new poster on this board but have been reading it for some time. You guys are great and provide alot of good info.
I love my 2003 SE.

 

the freeway blasters are loud, and seem to be popular, but if you wanted the loudest, could have gone with the Hella Supertone Electric Horns....at 140dB, i really want you to find something louder. everyone that's heard it on my car has asked for the link to buy it. i've already installed them on another max around here and 2 more guys want it done.

but it's also $65, so whichever is worth it.