Reverse Wire for Back Up Camera

ill like to start of my saying hi guys this is my first time posting but i have visited many times when i was doing my remote start alarm, there is a great community with awesome people! now on to my situation, im doing a back up camera in a chevy malibu 2008 ls 3.5l i have everything hooked up except the reverse wire i read there are two light grean wires in the back fuse box, to only thing i cant get to them, i built a sub-woofer box inside the trunk to use all the space available, so getting wires from the there is imposible, i also tried from the trunk lid but that is also blocked, i also tried to look for the dimming wire in the rear view mirror but my car does not come with that function, now with that out of the way i did find a light blue wire from the bcm that i guess works with the reverse relays, when in parked, driver neutral it reads 12 volts, when in reverse reads 0 (ground) is there a way to reverse the polarity with a relay? or im i screwed?    https://dl.dropboxusercontent.com/u/4191695/08_chevy_malibu_complete.pdf here are the wiring diagrams Fig. 43: Backup Lamps Circuit(light green wires, Fig. 11: Body Control Modules Circuit (1 of 4) light blue wire http://www.stevemeadedesigns.com/board/topic/190498-chevy-malibu-rebuild-4th-bandpass-trunk-150/ here is my car for reference, thank you guys :)

Catch it at the reverse lamp.

I wish I could get it at the reverse lamp the grommet where the wire exits the trunk and into the lid is blocked, that why I'm tring to tap into something before it gets to the trunk. Any ideas

so if Convert the Negative Output from the bcm to a Positive Output to my reverse wire it will work? im just scared because that wire shows positive voltage in park, neutral, and drive, i dont want to burn notting, sorry im just uneducated.

With positive on both of the coil connections the relay will not energize. When you put it in reverse and that wire goes to ground, the relay will energize. When the relay energizes it will then power your camera.

ok sounds good man ill hook it tomorrow and see how i did. just to verify wiring on a relay, trigger wire from BCM to pin 85, pin 86 and 30 to constant 12v+ with a fuse, and pin 87 to reverse wire in the head unit. im i missing something? really appreciate all the help, dont know what i would do with out you guys. for some reason wiring this was boggling my mind

How are you powering the camera?

acc wire.

Little update guys I did wire the relay yesterday night and it worked liked a charm! :D. Now I can pursue the instalation. Thanks guys :)