Other than getting a head unit with an aux port? No.
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: http://www.crutchfield.com/p_469GM12...2.html?tp=1672 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.