Well, Boudica, I got your fuel tank here. I’m really, really embarrased to admit this, but the thing is plastic.
Since your car is a 1996 model year, and that’s the year that the company changed to good old steel, but these things take time so I guess maybe they missed the wagons at first. (I would have ventured a guess and say you have a ZX3 or whatever the “sporty” model was at that time). This tank, though, would have been purchased from a subcontractor (probably Visteon), and not made directly by Ford (although maybe, since in this time frame the spin-off was under way).
I’ve taken a good, long, hard look at your tank and I’ll be damned if I can figure out where it’s leaking. Because it’s not a steel fuel tank (produced here), the correct fixturing for a submersive water test doesn’t exist for it, otherwise the leak would be super-easy to find. In this test, the tank would be pressurized with air inside of it, and submerged into a tank full of water. Any leaks would be readily apparent. But because the correct fixturing doesn’t exist, well, we couldn’t do that. Even if we could, I don’t know a thing about plastic, so I could find the leak but not have an explanation for it.
I realize you said the dealer indicated it was coming apart at the seam, but I have no evidence of this. Important: I’m not an expert in plastic fuel tanks, so I’m not saying the dealer is wrong or lied to you!
Despite that, I tried finding a leak, but there’s nothing obvious. There is, though, a very clean spot on the external bottom side of the tank – I wonder if the tech cleaned it for some reason, or if that’s where the fuel would have come from? Or maybe it leaked at the seam and ran down that way. You maybe can see the clean spot on the “FDM” picture linked-to below.
Regarding everything we’ve all talked about above, well, none of it applies. Plastic fuel tanks don’t rust, aren’t welded, and have no paint. They are either heat-welded (kind of melted together) or likely glued together (remember, I don’t know). Seems like a reasonable failure mode, especially if you’re running over cattle-guards down there in Waco all the time. 
Here are the pictures of your tank:
Bottom of the Tank FDM Retainer. On this one, I’m just guessing that the molded cup serves as the fuel delivery module (FDM) retainer. Aside from holding in the pump, fuel tends to collect here, which is nice in low fuel conditions, especially as you take a sharp curve. Mostly I provided this since it’s curious compared to steel tanks, plus the clean spots could indicate that gas was there. Unfortunately, I don’t know.
Well, I hope I didn’t get your hopes up, and I really am embarrased I didn’t know this was a plastic tank beforehand!
And remember: Anything I say is my opinion and not necessarily that of anyone I may or may not have any type of relationship with.