park brake interlock

Hi. Installed a pioneer avh-p3300bt in a 2002 Subaru outback last night and can't get the deck to recognize park brake input for the video interlock. I do not want to bypass this, I insist it must function as intended. The subaru only has 1 wire running to park brake switch and connecting to this wire doesn't seem to work. From what I could tell the wire seems to only have about 1.6V and the switch cuts ground when brake is off, provides ground with brake on. I wondered about using a relay but thought I would ask here if anyone has other ideas. I have installed these decks in the past with no issue, just stumped by the Subaru.

Thanks in advance.

I have never seen or heard of a parking brake wire that was anything other than 12 volts when the brake was not engaged. 

When you engage the brake, does it illuminate the brake light on the dash of the vehicle?

Any I've seen in the past have all been 12V as well. The dash brk light is working and comes on when park brake is applied. It might just be sending a signal to a BCM or something, running on limited wiring info for the car. I'm not 100% on what signal the pioneer deck is looking for, it says to tie the park brake wire to power side of the e brk switch but the only way I could get it to work last night was to leave the green wire unhooked, start a movie playing and once the movie started if I grounded the green wire the video would come on and stay on. Would it work to tie into the dash light? I'd really like to find a way to make this work, obviously I could install a switch but as noted I insist the safety is intact. Welcome any thoughts. Thanks.

Diagram greatly appreciated and I think I can handle that mission. It'll be next weekend before I see the car again to try this but I'll be sure to let you know if it works. Thanks for the help!