I think its coming from the transmission range sensor mounted right on the transmission. Look for a light green 10 pole connector on the drivers side of the transmission. The T41 wire (Yellow with a dark blue stripe) is labeled P/N on the wiring diagram. I think it grounds when in park or neutral. The other end of the wire goes to the powertrain control module pin 27 on connector 4. The powertrain control module is the one directly behind the windshield washer reservoir. C4 is the connector closest to the engine. Since the TRS feeds the powertrain control module directly, you can't just add a relay and expect everything to be all warm and fuzzy. You'll need some kind of high impedence interface to connect to it to prevent pulling the signal to ground inadvertantly. If you don't understand what I just said, its best not to mess with it till you do. However, it it makes perfect sense, have at it. But verify that is good information first. All info came from the factory service manual for a 2008 JK. I have not yet personally verified it.
08 4 Door Rubicon
Member of Two Trackers 4wd Club
Great Lakes Four Wheel Drive Association
United Four Wheel Drive Associations