It's simple once you do it once, but depends on your preference of specific behavior.
Your system most likely has a stereo jack if you're using any type of active preamp. The ring or sleeve terminal of the output jack is usually used as the switch to complete the ground circuit, with the batt negative and the preamp system ground leads going one to each. The active system engages the battery negative to ground when you plug a mono cable in, since the mono cable shorts the ring and sleeve of the stereo jack.
You wire the LED system's ground to the same point on the jack that the active system's ground, NOT the point on the jack that the battery negative connects to! This way, it will only monitor the battery when the guitar is plugged into a cable, behaving like the active system. You could also wire the LED system negative to the side I previously said not to, but install a momentary, normally open, SPT switch in between. This is the best way, IMO, since you can limit the current drain of the LED system to only moments that you actually press the button, so it won't have constant drain whether plugged in or not. The reason I say this is that these systems use an LED and an op amp comparator. Depending on the op amp used, it could have constant current draw of up to say 30mA, which one might consider unnecessary battery drain. You could check it by pressing the button once at the end of each set or gig.
The pos wire obviously goes to +9v on the battery.
Donovan, NH, USA
1998 Fender American Std. Strat
2008 Fender American Sp. Mahogany Strat HSS
2008 Fender Hot Rod DeVille 2X12
Last edited by Donovan; March 6th, 2010 at 03:53 AM.
Reason: Forgot to mention the +9v