I just realized how pathetically quiet our stock horn is when I had to use it a couple days ago. Now, I want to upgrade to a louder one, so I bought a Fiamm Highway Blaster (http://fiammamerica.com/Product.asp?ProductID=155). So I simply took off the stock horn and mounted this one in its place. That's the way it is now. It is rated at 130 dB and drawing 5A (am I risking blowing a fuse?).
How do I connect a relay? I bought one and it's got 5 terminals labeled 30 (fused battery power), 85 (ground), 86 (switch), and two marked 87 (lamp).
I referred to the wiring diagram (http://www.bbburma.net/Scans/Haynes_WiringDiagram_US_90-96.jpg) that Kerry scanned in.
I plan to:
(1) use this (http://www.allelectronics.com/cgi-bin/category.cgi?category=250&item=FHP-24&type=store) and a 10A fuse between the positive battery terminal and terminal 30.
(2) connect the black/blue spade connector that originally connected to the stock horn to relay terminal 86.
(3) connect the Fiamm horn to 87 and 87 with new wires and spade connectors (does it matter which connector attaches to which terminal?).
(4) connect the black spade connector that originally connected to the stock horn to relay terminal 85 (no fuse).
Does this sound right? I'm not very experienced in this area.
Fiamm horns are excellent; look on the horn for a ( + ) or ( - ) mark near the connectors. Though the polarity SHOULDN'T matter, no sense in smoking the horn if polarity IS a concern (there could be a diode inside the housing). I don't have any Fiamm horns laying around to check this for you, so I can't be more specific that that.
If the horn IS polarized (one side marked for Positive or Negative), attach that spade lug to the corresponding point on the relay which is directly across from the incoming line. IOW, if the horn shows one lug as Positive ( + ), wire that lug to the relay terminal which is directly across from terminal 30, the fused battery terminal, which as you already know, is the Positive power to the relay.
Other than that, if the horn is NOT marked for polarity, then it shouldn't matter which way you connect it to the relay.
From what you've described, you have everything else correct, so you shouldn't have a problem with the wiring.
When you test the horn, watch for smoke coming from the wiring and components, and remember to protect your ears. It would be best if you test everything BEFORE you permanently mount the horn; no sense in making more work for yourself if the horn happens to be defective.
Lastly, point the mouth of the horn slightly DOWN, so it isn't as likely to fill up with dirt, bugs, or rain as you ride. If you're not careful, you can destroy the Fiamm pretty quickly by allowing it to become an expensive bug-catcher. Taping a flexible (cloth, etc.) screen across the mouth of the horn will keep out the roadkill, and still allow you to sound like a Freightliner in traffic.