NewHD, has a very good point. According to the factory schematics for a 2009 FLHX (Doc, don't have 2008 prints) the stock factory horn has 2 wires. A Black wire which is ring terminal grounded to the right side stud under the seat. The other wire which (in 2009) is yellow & black, continues on (through a couple of connectors) and ends up at the left grip horn button. A color change takes place for the power feed going into the button. The other side of the horn button (power feed) is now orange/white. That orange/white wire goes to (& gets it's battery power from) the Accessory fuse. The accessory fuse gets it power from the ignition switch which must be in the ON position for the horn circuit to get power.
***In order to have battery power at the Horn, you need BOTH of the following ***
1) Have the Ignition switch ON and
2) Push the left grip Horn Button.
Important: First thing,,, test your test light tool to be sure it works and does not have an internal short. Clip the alligator clip to the Battery Negative Post. Touch the "needle point" of the tester to the Battery positive post. Test light should light and NO shower of sparks should happen.
If you pass the above "tool test",,,now perform the following steps to test the Horn Circuit.
1) Disconnect BOTH wires going to the Horn.
2) Take your test light and connect the red alligator clip to BATTERY positive.
3) Now touch the needle point end of your tester to the BLACK wire of horn wires.
4)Test light should Lite "on" indicating ground side of horn is OK.
1) Now take the test light alligator clip and clip it to FRAME ground.
2) Touch the needle point of your tester to the Yellow and Black horn wire. 3) Turn on ignition key and push the Horn Button on the left grip.
4) Light on tester should come ON when you push the left grip horn Button.
If the above takes place,, your horn is bad.
The 1st 4 steps Tests the ground circuit for the Horn.
The 2nd 4 steps tests the power supply Going to the horn.