100 bucks a piece! I paid 25 for the line out converter for the subs (Best Buy). The other adaptor could be a bunch of things... Basically, Ford used one of the worst setups I've ever seen of in a car. Because of the amps in the rear deck (2) and the one behind the head unit, they run high and low frequency lines to the amps and then run wires to the speakers. I had subs wired in to the rear deck speakers via a line out converter, it sucked... it sounded weak, even with a 800 watt rms amp, I could barely see my 10" infinity perfect move. There is a better way to hook it up, but I couldn't figure it out and the shops won't do it. The only good option for sound quality is to replace everything related to the stereo. This means that you must replace the headunit, speakers and run new wires to the speakers. Ultimate Electronics wants $100 bucks to rewire it. Plus 35 and 45 to install the speakers.
Here is ****'s way of hooking up subs to a stock stereo. This is the method I tried, but was unable to hear any sound. A shop will probably hook the line out converter to a standard speaker wire, as mine did, but it will sound like crap. I hope this helps.