Consider how much safer it is to sample the output directed to the turn signal, rather than the input to a $400 to $600 "body Control Computer" , as well as being much simpler to access, the drive signal to the actual turn signal light. The signal to the rear light may also double as a stop light, at least on some models. That is the other reason I do not suggest digging in to the switch wires.

 

Can I use the signal from the turn signal switch to the back to trigger the relay?

(This may be too technical, but if so, that’s the nature of the solution and you may have to look elsewhere.)

The problem is that switch is connected to the BCM and the nature of the BCM’s inputs is unknown. If you were to make a very high impedance, galvanically isolated connection (and not make any mistakes connecting it) you’d be fine, but it does require more than you might have expected to do.

High Impedance: a connection that requires effectively no current to operate. Impedance is effectively AC’s equivalent to DC resistance. This means that the connection is through such a high resistance it is all but invisible to the BCM‘s input making damaging or disrupting it is effectively impossible.

Galvanically Isolated: a connection that does not allow any voltage to pass between two devices. If the signal is AC, this could be using a transformer which makes a connection using a magnetic field.

For DC it can be a optoisolator which is a device that uses a light source (almost always an LED) on the side producing the signal to activate a phototransistor on the side taking the input. The phototransistor is like an ordinary transistor except that the base which is normally where a signal meant to turn the transistor on is wired is designed to use light.

The two devices (LED and phototransistor) are contained in a single light-tight package with a channel to allow the LEDs output to impinge on the phototransistor’s input. The isolation this can achieve is very high and it means that if anything should go wrong with your accessory lighting the electrical impact of that can’t reach the BCM.

It is the LED side of this device that must be contrived to present a high impedance to the turn signal switch.

 

"Y" is correct, but there is another concern as well. The nature of that input to the BCM is that the signal switch pulls it down to zero from whatever the pull-up voltage is from the BCM internal processor. That might be 3.3 volts, very common for current processors. If an external device operating fromthe 12 volt supply is connected, that would pull the input up to 12 volts,probably causing damage to the EXPENSIVE BCM. The suggested isolation scheme could work, but it is not really a simple circuit, it would require at least two IC devices and quite a few other components and a fair amount of electronic assembly skills. AND the project would reuire two of them.

 

One more consideration is that a suitable timer module is probably available for purchase, already built and packaged, ready to install. But depending on where you live, there may be separate concerns about vehicle additional lighting. Some areas are more like a Police State, where local authorities have nothing more important to do than seek out those who choose to be even slightly different. And given how bright those useful cornering lights were, as I recall, they will attract attention. They were rather distracting when an oncoming car was signaling for a left turn on an unlighted two-lane road.
So another choice would be a left-foot operated switch in a location like the older headlamp dimmer switch used to be years back.

 

In at least one recent vehicle I have driven, the turn signal was only triggered by the TS switch momentary action.

OK, in the one I drove just a few miles, about three months ago, the turn signal lever never latched in either direction, it always returned to the center position.

What vehicle is this? The TS is talking about a Chevy/GM. All of the GM cars have a "locking" turn signal switch. Although from some where around 1980 era they have a "passing signal" where you only slightly pull the switch lever to signal when your going to pass. They all return to the center, off, position after the turn is completed.

 

They all return to the center, off, position after the turn is completed.

Or manually if they don't.

 

Or manually if they don't.

I need to do that when I pull into my drive and get out to get the mail.

 

One more consideration is that a suitable timer module is probably available for purchase, already built and packaged, ready to install. But depending on where you live, there may be separate concerns about vehicle additional lighting. Some areas are more like a Police State, where local authorities have nothing more important to do than seek out those who choose to be even slightly different. And given how bright those useful cornering lights were, as I recall, they will attract attention. They were rather distracting when an oncoming car was signaling for a left turn on an unlighted two-lane road.
So another choice would be a left-foot operated switch in a location like the older headlamp dimmer switch used to be years back.

Actually, I am also adding a supplemental light behind the grill and planed to add a foot switch to control that.
what I would like to do would be to make the turn signal cornering lamps capable of turning on with th turn signals or with a switch. If I were to design a circuit, could someone tell me how to solder together my own circuit board?

 

I see fog lights on during the day and night for many drivers. evidently they believe that somehow it makes them safer. How does throwing more light into the eyes of oncoming traffic improve safety?????

 

I see fog lights on during the day and night for many drivers. evidently they believe that somehow it makes them safer. How does throwing more light into the eyes of oncoming traffic improve safety?????

In some states, that’s the law.

 

In some states, that’s the law.

I am not commenting about DRLs, (daytime running lights,), but actual FOG lights, positioned lower to shine below the fog. At least that is the theory. The purpose of DRLs is to help those who can not discern the difference between the front of a car and the back of a car discern which it is in front of them.

 

I have no problem with DRL's where the head lights are on but dimmer during the day, one method places the HL in series during the daytime.
Canada was the first country to mandate it for all vehicles.

 

but actual FOG lights, positioned lower to shine below the fog.

Again what cars? GM cars the fog lights can't be turned on without the low beam headlights being turned on.

 

@stryped does you Silverado have a center console? You could put a toggle switch in it a SPDT center off switch to control your lights. Toggle left for the left turn or right for right turn. Having both lights on would/could blind people in the other lanes. The toggle switch would operate a regular automotive relay for each light.