I have had 521 trucks for a long time, since the mid 1970's. One of the changes I did then was to change to halogen headlights, and I added additional relays to handle the current. I did several different ways of wiring additional headlight relay, and finally came up this way.
On a stock 521, the fuse box is the weak link in the wiring, and the headlights draw the most current of all the electrical accessories, the fusebox is barely adequate for the stock current demands.
This modification is one way of improving the performance on even stock headlights. It uses a second relay to take headlight electrical current directly from the battery, and bypass the fusebox, the headlight switch, the headlight wiring into and back out of the cab, several connectors, and about 5 feet of wire. It uses the stock headlight wiring that is bypassed to trigger the additional relay.
First, you need to get a relay, and supply power to it. This is how I supplied power to the additional relay. The orange wire goes to the additional relay, the white wire comes from the positive battery terminal.
This is a second picture of the battery, and the wiring to the fuse box. I made my own battery cable, it is a 2/0 cable going to the starter, and the white wire is a 8 gauge wire. The orange wire, with a fuse holder is a 12 or possibly 10 gauge wire.
This is a picture of the additional relay I added, the blue one. It was used in a different version of 521 headlight wiring before I did it this current way. You can use almost any relay, just make sure the contacts will handle headlight current. This is more critical if you use after market high wattage headlights. A good relay to use is a KC HiLites part 3300. This relay is rated at 40 amps, and has two 87 terminals, and does not have a 87A terminal. You can get them mail order from Summit Racing.
The stock 521 headlight relay has four terminals. You can identify the stock headlight relay by looking for a relay with three thick red wires, with stripes, and thinner light green wire, by the battery.
The additional orange power wire I added from the fusebox terminal goes to pin 30 of the added relay. You move the stock red with a yellow stripe from the original stock headlight relay to pin 86 of the new relay. Pin 85 of the new relay gets grounded, I used one of the screws that hold the original headlight relay to the inner fender. Then you make a short jumper that goes from pin 87 of the new relay, to the open terminal on the original headlight relay.
The red wire with a yellow stripe comes from the light switch, and has power when the switch is pulled out.
The red wire with a black stripe goes to the two low beam lamp filaments.
The red wire with a white stripe goes to the four high beam filaments.
The coil inside the stock relay is is hard wired to the terminal that gets power from the light switch, on the red with a yellow stripe wire.
The light green wire, with a red stripe goes to the turn signal, and is grounded when the turn signal lever is pushed forward.
Yes, I know the blue relay is just hanging by the wires. It should be mounted to the inner fender, but the wires going to it hold it in place pretty well. The KC HiLites 3300 relay does have a mounting tab on it.