I installed an Alpine iLX-107 in my 2008 EX-L. Rather than track down the parking brake wire I went with a micro bypass switch, a $12 option. For yours search for "Sony bypass switch" on Amazon.
I had installed an iLX-107 in my 2005 EX before installing one in my 2008. For that one I did hook up the parking brake wire. However, I found that often I had to engage the brake multiple times before the unit would recognize the connection. The connection was solid so maybe it was something with the Alpine. Anyway, I replaced the connection with a micro bypass switch. Problem solved. I like that I can access the unit's settings without engaging the parking brake.
For the Alpine if the brake wire was grounded when the unit booted it wouldn't recognize it as having the parking brake engaged. To work one would have to ground the wire after the unit booted. The micro bypass switch handled it, no problem.
some head units are easy in that you can simply ground the green wire...others have to sense the actual change in no ground to ground. For my pioneer I had in my 09 I found a company on ebay who made a simple prefab bypass for like 11 bucks shipped. 3 wire install and never had to do anything else after that for it to operate. For your head unit I found similar product on amazon...