Hey Nick, this is Nam. I was there when you bought the IA20.1. I finally realized who you were LOL.
On your cousin's headunit, it has 2 RCA preouts, but none that are dedicated for subwoofer duties. If you look to the left of that page you'll notice in the "General" section that it has 2 preamp outputs, but just below that it does not have a sub preamp output. You would want something with a sub preamp output for your IA20.1. Most radios have 3 RCA outputs, 2 for the 4 channel amp (front and rear), and the 3rd would be for subwoofer duties. I think you can find an Alpine CDA-9856 for a fair price if you needed something.
Double DIN is like having 2 radios stacked on top of each other. This is a picture of my Pioneer FH-P4200MP that I have laying around. Sucker was only used for a week then the car got totaled //content.invisioncic.com/y282845/emoticons/crying.gif.ec0ebefe590df0251476573bc49e46d8.gif.
The wierd thing about this radio is that it has only 2 RCA outputs, but one is for the front, the other is for subwoofer duties. //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif