EDIT: I lied lol. There are antenna adapters that allow you to switch between the antenna and an aux input -- they basically inject the sound into the antenna input. Same method as the PAC harness, just different point of injection. I personally use this one: Precision Interface Electronics GM12-AUXv2 Connect a portable music player, satellite radio tuner, or DVD player to your factory GM receiver at Crutchfield.com but I got it cheaper than that. I believe Crutchfield had it on sale at the time. Also, I don't have the Monsoon system so I'm not sure if it'll work with that particular head unit. You can always check on Crutchfield though.
Yea that adapter should work, but it says something about getting the radio programed to recognize the adapter.
04 GP GTP Comp G - Paint mod= +50hp
00 Ford Focus zx3, 95 VW Passat Vr6
92 Chevy Silverado - 305 w/ 5-speed - rust mod
80 Cadillac Eldorado Too Many Cars!!!