Ahh ok, I gotcha. Your problem is you don't have the brace
You're gonna need to scour around ebay and stuff to find one- the brace you're using right now is for a 2 din, where the brace that other people have is made for 2 one din slots. So when you take the WRX stereo out of there- you don't have the correct din brace.
I might be able to help you- seeing as how I'm moving my head unit somewhere else. To be quite honest tho, I'm not sure about that. If I end up not needing the bracing, I'd totally be willing to sell it to you- but if I need to cut it up and stuff... :/ anyway. You want to check ebay, and check
www.6gc.net buyer/seller forums (put a WTB request up there) and see if they can help you.
Once again, I believe this is your problem, but pictures would help!