This DIY will get you started with some custom light effects for your rear fog light. It will flash when in reverse. It will light solid with the brakes (unless in reverse), and you will also have the ability to completely turn it off or completely turn it on. First off are a few thanks and credits. Simota1 with the original Fog Lamp Install. M.J. with the rocker switch source. And AK370 with the Fog Light with Harness Install.
So here are the parts that you need. You'll need two of the sockets and two of the relays. For the rocker switch you want the one labeled Full Size Rocker Switches - 1/4'' Tabs. Get the ON-OFF-ON SPDT version so you have two separate on positions. These are just samples, you can pick your own brand if you want. And you can replace the flasher shown with an adjustable one for a faster flash.
Parts-Express.com:12 VDC Automotive 5-Pin Relay SPDT 30/40A | spdt relay automotive 12v bosch alarm
Parts-Express.com:12 VDC 5-Pin Relay Socket | 12v 12 volt bosch alarm relay relay socket remote start starter kill
Chevs of the 40s: 5297633 ALL+ - Chevrolet%20FLASHER SOCKET W/ MOUNTING BRACKET
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Rocker Switches
Now here is how the pins on the relay are numbered:
The following is what you need to wire up and what pin to wire it to.
Relay 1 30 ----> 12V Switch 1
Relay 1 86 ----> Ground
Relay 1 85 ----> Reverse (orange taillight harness wire)
Relay 1 87 ----> Flasher In
Relay 1 87a ----> Relay 2 30
Relay 2 30 ----> Relay 1 87a
Relay 2 86 ----> Ground
Relay 2 85 ----> Brake (green taillight harness wire)
Relay 2 87 ----> Fog Light +12V
Relay 2 87a ----> Not Used
Flasher In ----> Relay 1 87
Flasher Out ----> Fog Light +12V
12V Switch 1 ----> Relay 1 30
12V Switch 2 ----> Fog Light +12V
This is much easier than it seems as listed here. When these parts are sitting in front of you they are labeled by pin number and all and it is very easy to see how to wire them together. As soon as my rocker switch comes in I will post a video of the operation. Good luck.