The 1970 Mustang wiring, as well as other years of Mustangs, can be somewhat less than adequate for the operation of the stock headlights, much less running the newer Halogen lamps. Often the headlights can seem dim compared to today's cars. This can easily be corrected and will ensure longer life of your lighting system components such as the headlight switch, dimmer switch, and wiring. In the factory setup, power feeds the headlights through the ignition switch, headlight switch, dimmer switch, and finally the bulbs through the factory wiring. If age has in any way lowered the efficiency of any of these components, dim light and component failure can result due to component or wiring overheating. What we have done here is install Bosch type relays to handle the battery voltage needed to power the headlights without straining the headlight, dimmer, and ignition switches. The headlights will get the full 12v power without going through the factory system that can lower the voltage available resulting in dimmer lights at night.

In the schematic below, the wires are shown with the primary colors to help you understand which wire in the main harness is to be cut and spliced into. RED-GREEN refers to the red wire with a green stripe and so on. The blue wire from the starter solenoid can be any color you wish. A larger gauge of wire such as a 12 or 14 gauge will carry the load better, especially if you choose to use halogen bulbs. Some installations even use a 10 gauge wire from the battery. Just be sure you install the 20 amp inline fuse before you get to the relays. The relays can be mounted anywhere you wish out the way and out of sight. You might even want to just mount them side by side on a small board, painted and mounted under the hood somewhere in the engine compartment down in the corner behind the drivers headlamp. The headlight harness will have to be unwrapped at the brown connector in front of the radiator far enough back so you can see which wires are coming from the firewall connector and which wires go over to the passenger side headlamp. You want to make the cut and splice at the wires coming from the firewall connector. Connect the relays as shown. You will have to add wire at the point you cut the main feeds to reach the relays. In this type of connection, you can either solder and heat shring the connections or you can strip the wires back about 1/2 inch, twist the wires together tightly and wrap with electrical tape. Crimp or "butt" connectors can be used however they can be bulky and make rewrapping not look as nice. There is not much pull being put on this particular application, therefore simply twisting together and wrapping with electrical tape should be adequate.

OPERATION:
With this modification, the relays act as the switches for the headlights, sending power directly from the battery to the lamps. The factory switch setup simply acts as a switch for the relays, sort of a switch for the switches. All the factory setup does is energize the relays when you want to turn on the headlights and the relays then perform the actual lighting of the lamps, bright or dim. The factory dimmer switch still does it's job, energizing the proper relay, bright or dim, that you desire at the time. Your headlight switch still functions as it always has only now the load for the headlights only goes through the relays directly from the battery . The inline fuse shown is to ensure power overload or a short won't damage anything.When power is applied from the factory setup to relay pin 85, it grounds to the chassis and internally connects pins 30 and 87. Power is always on pin 30 from the battery, so when the connection is made between 30 and 87, battery power lights the lamps. When you have the headlights on DIM, the DIM relay operates. When you put the headlights on BRIGHT, the BRIGHT relay operates. That's all there is to it. Now you can operate your headlights in normal fashion and the factory lighting components won't be stressed by the load.

 
Parts needed:

- Roll of electrical tape
- 2 Bosch type relays
- 1 inline 20 amp fuse holder and fuse or breaker
- Main feed wire from starter solenoid to relays
- 2 lengths of wire to connect BRIGHT relay
- 2 lengths of wire to connect DIM relay
- Ground wire for the relays to frame

 
 
Below is a brief list of some sources for headlight relay kits that can be added to the existing wiring harnesses.