Having recently experienced this, I'll add a few things that may help.
First of all, your primary reference should be this:
99M03 Recall Information
Even though the guy(s) at Ford said that you need a new Wiring Harness, it may not be completely neccesary. While chances are your wiring harness is in terrible shape in some places, does it still work? If you are not suffering from shorts, unexplained battery drainage, or poor performance in general - You may not need to replace it. The only reason to do so would be your peace of mind on the vehicles dependablity.
My Harness Story:
I took my 95 Contour with a V6 into a local Ford dealership roughly a Month ago. It took them about 13 days to get the Job done properly. I have a sneaking suspicion that the electrical guy was only there every Mon/Wed/Fri as that was when their next estimate always fell on when I harassed them about it. But all that time aside keep in mind that this was a qualified technician, who's done this before, but probably took his sweet time finishing it.
I spoke to the guy a few times whenever I stopped by and he explained to me that it was a pain in the ass to do, and I have no doubt of that.
So, after 13 days and 2 rental cars later I get it back and a number of things were wrong with it, but I'll give the guys credit in that whenever I brought it back they would try to accomodate me.
The problems? CEL#1 due to vacuum leak, CEL#2 Loose fan connector. Few parts they "forgot" to put back on. (Air box clips, battery tie down, etc) Nothing life threatening but stuff that should have been checked in the first place.
The other problem was the guy didn't route the wires properly. It would have been nice but I can see why he didn't, as long as the car works he did his job I guess. So I've spent the past two weekends under the hood doing my best to put things back into place, which for the most part I was able to.
Well that's my story so lets move on into yours...
If you read the 99M03 Recall sheet, it gives you a basic step by step procedure as to what you need to do. Some of it easy, most of it not. I'm basing this in the V6 description but most of the work is disassembly/assembly in order to route the cables around the engine along the firewall/left fender/behind the radiator/right fender. The major parts are:
1. You need to remove your Upper Intake Manifold, included with this is your Airbox --> Throttle body assembly. You need to do this to access the Fuel Injectors.
2. You will need to remove some of your coolant hoses and drain the cooling system in order to put the Harness in place without trying to snake it through. The harness is not flexible at all so removing components is a neccesity.
3. Replacing the Fuse box under the hood with the new one, moving your old relays into the new box.
4. Moving lots of pieces around, Coolant resevoir, Brake fluid resevoir, and anything else in your way. My Front Bumper, Drivers-side-wheel-well-splash-guard, and the plastic beneath the windshield wipers were removed as well.
5. You also need to connect one end of the harness to your interior fuse box. The plastic piece behind the Hood release that runs along the floor needs to be moved to access those wires. This is an assumption as I had to connect my dome lights as Ford neglected to, as well as reposistion a few wires. Someone had torn up a non-important piece of the firewall up there to (I guess) route new wires to the Interior.
Long story short, if any of the above stuff is questionable to you - You may not want to take this project on. At least if you don't have a backup vehicle to get around in as it will take some time.
When I was properly re-routing the wires underneath my hood I had to unplug a number of them to move the cables around. Alot of the connecters were similar so I imagine without a good reference with a lot of pictures it would be difficult to tell what connector goes where as none are labeled. Some are easy to guess though as they are unique. The harnesses themselves are very rigid so there placement is important. Also there were about a dozen or so left over connectors on my wiring harness(es) that I had to tuck away into the corners and the only options not on my car that I can think of was Traction control and the non-applicable tilt steering.
So..... To end this. It is a massive project - Best summed up as just short of dropping the engine.
If you read through the 99M03 list and spend a good hour or two looking at your current harness and where it goes and what it passes through and still feel comfortable with it. Go for it.
I realize this post is a bit pessimistic but realisically if I hadn't still been under warranty, I would probably still be using my old harness as the only problem I ever had was when I washed my engine with a lil'

bit of water. It didn't work but hey, It sure was shiny!
Hope this helps and sorry for the length.

/self