I'm installing a new head unit and backup camera and am looking for a decent, in-cab reverse circuit location to tap into -- specifically so the head unit knows to switch to the backup camera when the vehicle is placed in reverse. I'm desperately trying to avoid going through the firewall and into the under-hood fuse block (where I know I can tap the reverse circuit), as it's a royal pain. There's got to be one... This is in a 2004 GMC Sierra 4x4. Any pointers would be appreciated.