Timing,
Let's take a stab at this.
First, If you've put the timing light on the #5 wire and you can see that it's not firing, it certainly looks like you're in the right direction. You could always swap wires with the cylinder next to it. (JUST TO BE CLEAR HERE.....swap both ends of the wire. Move the bad wire IN ITS ENTIRETY, to a known good plug and retest) You shouldn't worry about the injector, it has no idea about the firing.
The plug wire could be the problem, but I'd suspect a bad coil. I've got a 2000 Windstar w/ 3.8l and an alldatadiy.com subscription for it Unfortunately, what I'm out of town and don't have ready access to my van and my subscription just lapsed, so what I'm about to tell you is strictly from previous experience on other vans and somewhat off the top of my head.
The coil packs are probably three coils that each have two contacts. So, just for discussion sake, lets say that the coil packs are bunched like this (1,4 & 2,5 & 3,6). As the engine approaches firing time for cylinder 1, it's going to fire both cylinder 1 and 4. It doesn't matter on 4 because of the pistons location in the combustion cycle. The connector coming from the computer to the coil pack is probably a four wire connector. One is going to be 12v constantly. The other three momentarily ground in sequence. As one wire grounds, it will fire it's respective coil packs and then immediately return to a high (non-ground state).
Using what I've discussed, you should have some new troubleshooting to do. You should check the spark plug wire that is on the same coil as your suspected bad one. If it's good, you can do some shadetree mechanic-ing here. Put your known good wire on the other side of the coil pack. (the one you just tested) Leave the known good wire disconnected (you don't want to misfire multiple cylinders at the same time...I'd rather simply misfire one and have the other not fire at all) Your car will run very rough because you are down to 2/3 of your available cylinders, but you should definitely be able to check your timing light to see if you are getting fire to that cylinder.
Bad news for you is that if any single coil goes bad, you'll have to replace the coil in its entirety. If you find out that the coils are good, you'll need to move toward the computer to troubleshoot further.
Now my final disclaimer....everything that I've offered was off the top of my head and from my experience working on a Chrysler van. Perhaps Ford did it differently, but I doubt it. Good luck and let me know how it turns out.
Mark