Well, I am not a diesel mechanic but I do know my way around Electrical.
Here is what I would do.
On the 2 big terminals of each relay, one will be +12v power all the time and one will read ground when the relays are off and the glow plugs are off and SHOULD read +12v when the glow plugs are on.
Now if the relays were faulty, the glow plug side will likely never read +12v.
As far as the small terminals go. I don't know if Ford uses negative or positive to engerise the relay. Either way one small terminal will read negative when the glow plugs are supposed to be on and one will read positive. Now if both read either positive or negative that likely means that either the controller for the glow plugs is shot, or there is an open circuit somewhere or a fuse or fusable link blown.
Test it out and let me know what you come up with and I should be able to provide more insight of what is going on and why. You might have to do more testing at that point as well.
Again I am not a diesel mechanic but doesn't ford use a preheater? If so, would that be what one of the relays is for?