The wire is quite small (#18) and it follows a very long torturous path from the battery to the headlight switch, down to the dimmer switch and then back thru the engine compartment to the headlights. There is several volts lost thru all this wiring and switches, so the lights only get maybe 10-11 volts instead of the 13-14 they should have.
The other problem is that all that headlight current heats up the headlight thermal breaker, which eventually dies from the well known "flashing Ford headlights" syndrome.

Do not attempt this without the beer.

I drilled 2 holes in the fender, between the battery and the starter relay and screwed the relays in place. Label one of them "high beam" and one "low beam".

I already had a nice fuse block mounted in the vicinity with extra slots available, so I used it.
The rest of You will need to mount a 2-fuse block somewhere near the battery. Connect wires from one end of each fuse holder to the battery using some #14 wire. The big terminal on the starter relay is a good place to attach the battery end of these 2 wires.
Connect the other end of a fuse holder to one of the "contact" terminals on the "low beam" relay using #14 wire. If You use the Bosch type relay, the appropriate terminal will be labeled "30".
Connect the other fuse holder to the "high beam" relay, just like the first one.
Connect a short length of #18 wire to one of the coil terminals on the low beam relay. On the Bosch style relay this will be "86".
Ground the other end of this wire to a suitable, clean chassis ground.
Repeat for the high beam relay, grounding coil terminal "86"

Now take out the drivers side headlight. Remove the trim piece around the headlight bucket so You've got some room to work.
Remove the bulb from the connector and cut the wires (red, green, and black) loose from the back of the connector. Leave an inch or 2 of wire on the back of the connector so You have some options when we get to a later step...
Tape up the cut ends of the 2 wires that dangle from the harness.
(red and green) We Aren't going to use them anymore, but they will still be hot when the lights are on, so make sure they're insulated and can't get in trouble.
Unscrew the black ground wire from the back of the bucket.

Now You get to decide how to handle the bulb connector.
I see 3 choices:
  1) Reuse the old connector, crimping or soldering a #14 wire onto each stub of #18 wire left on the back of the connector and insulating with shrink tube/tape.

2) Buy new headlight connectors, available at the parts store and crimp or solder Your #14 wires onto these.

3) Pop the spade lugs out of the old connector, cut the wires off and solder Your #14 wires directly to the old lugs, insulate with heat shrink and pop them back into the connector body.

I chose option #3 as being the best for me...

Once You've decided, attach a #14 wire to the high beam terminal (red) and another to the low beam terminal (green) of the connector.
Make them long enough to route nicely from the headlight bucket to the relays, plus enough extra to get the light out easily for servicing...
Attach a #14 wire to the ground terminal (black of the connector. This one only needs to be a foot long. Put a ring lug on the other end of the ground wire and ground it in the original location. don't forget to clean the metal all bright and shiny first. Smear a little silicone grease on the metal to prevent rust would be nice too..
Route the wires up to the relays, keep it neat and use some tie-wraps to keep things secure. Now You can put the light back in place.
Oh wait, if You put some silicone grease in the connector for the back of the light, You'll avoid trouble later.. Now put the light in.

O.K., so now You've got 2 #14 wires from the left headlight up to the relays. put a 1/4" crimp lug on each wire and plug the high beam wire onto the unused contact terminal on the high beam relay. On the Bosch style relay this is terminal "87". There are actually 2 terminal "87"'s on the relay. Use either one, they are connected together internally...Now connect the low beam wire to the terminal "87" on the low beam relay.

O.K., so You've got the drivers side light grounded and wired to the relays #87 terminals with #14 wire and the light is back in, right? Good, lets continue...

Pull out the passenger side light, and cut the connector loose from the harness as before, but be careful to leave the harness wires long enough to connect some #18 wires to them. We're gonna use these to drive our relay coils. So, crimp a #18 wire onto each of the 2 lighting wires poking out of the harness (one red, one green) and route them up to the relays. Connect the high beam one (red) to the coil terminal on the high beam relay. On the Bosch style relay this is terminal "85". Connect the low beam one (green) to the coil terminal "85" on the low beam relay.

Now connect some #14 wires to the pass side headlight connectors, and ground the black terminal and route the red and green ones to the relays just like we did for the drivers side.
You remember how we did the drivers side right?
Hey, go easy on those beers now!
Hook the low beam wire to terminal "87" on the low beam relay, and the high beam wire to "87" on the high beam relay.

Alright! that's all the wiring!
Silicone grease Your pass side headlight connector and put the bulb back in and any trim back on. Make sure all Your wiring is secure and can't get in trouble and then clean up Yer mess...

Now You can stick some 20 amp fuses in Your new fuse holders and hook the battery back up.

Test out Your new handiwork and enjoy!

There are 2 rudely drawn schematics that go with this.
One is "stocwire_1.jpg", which shows how the lights are more or less hooked up stock. The other is "newwire_1jpg", which shows the new wiring, with the original wires in red and the "new" wiring in blue.
Also, shown in green is wiring for an added switch so when the high beams are on the low beams are also on (controllable with a toggle switch), and a "high beam flash" option that allows You to flash high beams with the headlights off. (momentary pushbutton switch).