IIRC from my Stereo Advantage days, Audiovox had something that is similar to the thing that you apparently bought and wired in, but it has an analog 1/8" male cable coming out of it (snake this thing out from under the dash, into a center console, wherever your little heart desires) and you can plug it into anything that has a 1/8" output.
EDIT: i think its this thing…let me actually read the website lol and i’ll get back to ya
http://www.circuitcity.com/ssm/Customer-reviews-for-Audiovox-FM-Modulator-FMM100A/sem/rpsm/oid/80974/rpem/ccd/productDetailReview.do
EDIT #2: yeah thats the thing, its for a CD changer, but they (Audiovox) makes an adapter cable to get you to the 1/8" analog plug