The decomp solenoid only fires for a split second. If you want to make the light turn off, maybe have a look at the 05 electrical diagram & see how they do it.

I did something simular on my '02. Got a 12 volt relay at Radio Hack and wired it to the starter motor side of the starter solenoid. When you hit the starter button the juice going to the starter motor opens the contacts of the relay and turns off the headlight, I have the power going through the NC side. I jambed the relay into the headlight bucket, there isn't a lot of room in there and if you use the headlight ground as the control wire it will fit better.. If you don't use the ground you will need to run the low and high beam wires through the relay which is what I did. You might want to pick up an extra headlight socket with the plug on it. Suzuki didn't waste any wire between the socket and plug. I used spade connectors on the relay which used up most of the wire length, if you solder the connections there will be plenty of wire to get the job done. Take a look in the headlight first, you'll see what I'm talking about.