Hey all I am currently creating a custom rain light for my car (Light on rear bumper that flashes 5 times then holds when brakes are pressed). At the moment the rain light has positive and negative connected straight onto the + and - of a brake light globe wiring. However it being super bright and only wanting it to really be used in rainy or reduced visibility I wanted a way to be able to turn it on and off with the rear fog light switch in my car. The rear fog light also has only a + and - going to it. How am I able to have it so if the rear fog light switch is off pressing the brakes does not light up the rain light but when the rear fog light switch is turned on it then will flash when brake pedal is pressed. I'm not sure how to really go about this as wiring up something like this is completely new to me and I'm not sure if its really even possible or not.

 

Forgot to mention its running 12V as well incase that wasn't normal or anything for car wiring

 

The easiest way will be to use a 12V relay that is operated by the switch and turns on both the fog light and the brake light. There may already be a relay involved, so check. But a double pole relay will do the trick.

 

The easiest way will be to use a 12V relay that is operated by the switch and turns on both the fog light and the brake light. There may already be a relay involved, so check. But a double pole relay will do the trick.

I'm honestly so new to this I'm not 100% on how to wire something like that. I've got some basic knowledge and if there was a guide on how to do it I can follow easily. With the double pole relay will that only allow the rain light to flash when brake lights have been turned on IF the fog light switch is on? Sorry if I don't really understand I'm just getting into this sorta stuff. I'm watching a video on the double pole relay but still confused

 

It is illegal to tamper with vehicle lighting.
Vehicles on roads use a rear flashing light as a turn signal, not as a "rain?" light.
Your "fog" light is used as a backup light. The backup light is never turned in when the vehicle is driving forwards.

 

It is illegal to tamper with vehicle lighting.
Vehicles on roads use a rear flashing light as a turn signal, not as a "rain?" light.
Your "fog" light is used as a backup light. The backup light is never turned in when the vehicle is driving forwards.

My fog light is my fog light. I have reverse lights for backup lights. I can turn my rear fog light on whenever I like. my backup light is when I go into reverse. This flashing light is red to match my brake lights. It flashes 5 times when my brake pedal is pressed then holds solid until my brake pedal is lifted. This rain light is for people to see me easier when I'm braking for when it is near zero visibility. Hence why I want it on a switch as it will NOT be used all the time and only when visibility is near ZERO

 

It is illegal to tamper with vehicle lighting.
Vehicles on roads use a rear flashing light as a turn signal, not as a "rain?" light.
Your "fog" light is used as a backup light. The backup light is never turned in when the vehicle is driving forwards.

It also isn't illegal to "tamper" with vehicle lighting where I am. As long as it follow the road rules it is legal. Hence why I'm putting it on a switch and not constantly on as that matches my country's road rules.

 

I'm honestly so new to this I'm not 100% on how to wire something like that. I've got some basic knowledge and if there was a guide on how to do it I can follow easily. With the double pole relay will that only allow the rain light to flash when brake lights have been turned on IF the fog light switch is on? Sorry if I don't really understand I'm just getting into this sorta stuff. I'm watching a video on the double pole relay but still confused

A relay is just an electromagnetically operated switch. If it is connected to the existing switch, it will operate when it is turned on. The fog light is connected to one set of contacts and the rain light to the other. If the rain light gets it’s power from the brake lights, then the switch simply either connects or disconnects this. The same is true of the fog light which is connected or disconnected from its power source.

Here’s some information about a relay, it is a DPDT in this case, you can ignore the NC (normally closed) contacts, they don‘t matter to you. http://www.learningaboutelectronics...a-double-pole-double-throw-relay-in-a-circuit

 

Using a relay. The relay is activated when the brake pedal is pressed but the Rain Light only comes on if the Fog Light is also ON

 

Using a relay. The relay is activated when the brake pedal is pressed but the Rain Light only comes on if the Fog Light is also ON
View attachment 236673

Could I use this relay? Could i connect foglight + to #1 and - to #2. Then the cut the + to the rain light connect it to the #3 spot then out on #5 so that will continue to the rain light if that makes sense? That should then make it so when the foglight gets the power it will switch the bar from #4 position to #5 then allowing the power to flow through whenever the brake pedal is pressed?

 

Using a relay. The relay is activated when the brake pedal is pressed but the Rain Light only comes on if the Fog Light is also ON
View attachment 236673

I just realised what i posted might have been literally the opposite of what you said. Ive used a circuit simulator online and they both seem to work. Will there be a downfall using one of the other?

 

OK, I see what you mean, like this:
Either way will work OK.

 

If you wire it your way won't the Rain Light always be ON when the Fog light is ON, how will the brake pedal control the light?
OK, I see what you mean, like this:
Either way will work OK.
View attachment 236676

Yeah exactly like that, thank you so much for the help! I was worried I wouldn't be able to figure it out with such little knowledge but im glad I've got it half way done. Once I wire it up and everything I will post here to confirm i actually did it correctly. Thanks again ) was a big help

 

Got it all wired up, works perfectly. Thank you all