Oh hey!! I didnt know you posted. I usually look for the towncar forum folder to be lit up, i must have missed it the other day, sorry about that. I assure you i was not ignoring you. If I forget to check feel free to email me, send PM whatever.
If you buy the bypass harness it will plug into the black plug at the stereo controller in the trunk, along with the antenna. It will extend the wiring 18 feet long so that you can run it up to the front of the vehicle. All it is, is a plug in extension. Since the display in the dash has no amplifier or sound processor in it, all of that is done in the trunk. All you are doing is picking up all four speakers and antenna and running them to your new radio because it is self contained.
That harness in perticular is from a taurus. Same radio but different display. In a taurus you have to do the same because the radio in the dash is just a couple lights and buttons, the actual radio parts are in the trunk, since the new radio contains all of those, you need to run speaker wires from the dash to the easiest place to pick up antenna and speakers, which other than at their respective locations is in the trunk at the plug.
The plug is the same output plug that has been on all premium sound ford and lincolns since 86 or so. They reuse that same plug in many applications such as keyless entry, the computer in my digital dash and so on, Makes it easy so you dont have to reengineer a plug. So they use the plug from the old premium sound amplifiers. that means you can use a regular ford jbl premium sound bypass harness to plug in to it. If you unplug the black plug from that controler in your trunk, that is your speakers. Pretty much what you are doing, is anything that can plug into that for ease of installation, and extend it 18 feet to where you need it, aka the dash.
As for the antenna it is just easier for ford to run from the antenna to the controller, than from antenna to dash. Since you also need that in the new head unit, run that up to the front. Since the 18 foot antenna adapter is standard size you need an adapter in the trunk.
Keep us posted. Sorry about the silence.
Big Joe