Not the same one, but a similar piece, I believe it was made by Clarion. Most of the universal ones use a switch.
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
Bah. Haha I hit quote just a second too late. I didn’t think a mustang would fit under my dash. 
I’ll look that over. Thanks!
yeah i noticed that as soon as i posted, please forgive me i am trying to do about 4 things at once right now…i must have ctrl + C the wrong page…should be the right one now lol
oh, and if in doubt, im sure you could just go to Stereo Advantage (or circuit city seen as how they seem to carry this thing) and ask one of the installers. I know we used to do these installs all the time, for this specific purpose.
Now that looks like it would work. Only $50, same price as everything else. Only difference is that I’d have to splice for power, but that’s no big deal. Hmm…