I wasn't going to post as this was getting complicated and going over my head when trying to figure it out on my own. I enjoy AC/DC electrical and DC electronics, but circuitry makes my eyes glaze over when the math begins. I can partially read an electrical schematic but can't get much further than Ohm's law, or remembering parallel vs series....so I'm at a pretty basic level. I can google, so there's at least that going for me. I try to reference images or youtube whenever possible.
I'm looking to install some OEM rear fog lights on my wife's new vehicle. I was going to wire them up just as brake lights, and swap the incandescent for LED if possible (working on socket compatibility now). I stumbled across this forum in my searches, to this post: https://forum.allaboutcircuits.com/...filament-bulb-as-a-dual-filament-bulb.141090/
I can solder, and have wired LED's with resistors many times but diodes are new to me. I get the concept, but unsure of any specific values.
I've wired up relays for fog lights in the past, but always thought they had a specific wiring method. The above link is confusing, but I digress. I've never had much luck with relays having great longevity. I looked at Fujitsu, Schnieder Electric and Hella brands. Fujitsu are supposed to be reliable, but most are not multi-use. Schnieder have 6-10 pins so not exactly basic, and Hella has a solid state relay floating around, but I've only found a 4 pin version. In the above link, ian stated:

What that sounds like would be easier with an LED unit and a .

I attempted to search the forum for "programmable current source" and most posts are involving applications that I can't logically apply to my case.
If someone has a simple way to go about what I'm trying to achieve, I'm interested to hear it. - otherwise, my plan as of now is to just simply run these two-wire, single filament (soon to be LED) lights as basic on/off brake lights.

Thanks in advance.

 

What is it that You are trying to achieve ?

Generic LED Tail-Lights are readily available, and reasonably priced,
and come in a wide variety of sizes/configurations.

The only problem that I can think of is tying them into your existing Wiring-Harness,
and even that might not be an issue if You have an SUV or Pick-Up-Truck with
a factory installed Trailer-Lights-Plug.
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My 2022 Toyota Highlander has rear bumper reflectors. The European highlander models have rear facing, red fog lights instead of reflectors. I planned on getting them as additional brake lights. I have the part numbers for these OEM European lights and plan on ordering them. They use a single (T10?) style incandescent bulb. My model vehicle does not have a tow package to spice into, so I'll be ordering the factory tow harness to avoid splicing the stock wiring and get a brake light signal. The Euro models actually have a dial on the turn lever (with dash indicator) to activate these lights during poor visibility similar to front-facing fog lights. I'm just looking to run them as additional brake lights, but if I can somehow add parking/running light signals to these bulbs as well it would be nice. If it's too crazy, I'll just stick with plan A. I'm avoiding aftermarket soldered LED setups as the bulbs are not easily replaceable if something fails. The OEM's just have a single plug-in bulb.

 

I'm just looking to run them as additional brake lights, but if I can somehow add parking/running light signals to these bulbs as well it would be nice.

Are you trying to use the same bulb for 3 different functions?

 

In some countries it is illegal to tamper with vehicle lighting because many people get the wrong brightness, wrong color and wrong beam angle.

 

Two functions if possible. 12V (w/resistor) for dim running light, 12V (no resistor) for brighter brake light.
I've checked my local laws. As long as they do not flash and are red, additional brake lights appear to be unregulated similar to a 3rd brake light. No laws requiring them, no laws prohibiting them. I'm not tampering with the factory (required) lighting.

 

There are "retro-fit" LED replacements for all common Automotive-style Incandescent-Bulbs,
but, of course, there are plenty of el-cheapo Chinese versions too.

Spend at least several hours searching in Automotive-Forums to find
the best performing brand and model of LED replacement Bulb.

In the USA, just go to any NAPA-Auto-Parts-Store, they don't sell any low-quality parts.
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I've researched this quite a few times over the years. I replaced all lights on my Tahoe except turns and same with the wife's old car except turn/park/brake. All Sylvania (and some Philips where Sylvania didn't fit). As far as the Highlander goes, I plan on going all LED as well. Headlights, taillights, as well as front map lights are factory LED. Many run a brand called Lasfit (CANBUS) but the turns appear to overheat and hyperflash after a few minutes. A company called Auxito (CANBUS) appears to have better cooling designs and don't have this problem. I'd prefer Sylvania or Phillips, but for simplicity I may just run the Auxito for turns. Undecided about the rest. These OEM European rear fogs use a bulb 81210c, which Is a Toyota part number, but US Toyota websites don't come up with results. It's difficult enough just tracking down these European lights. The bulb looks like a T10 (194/168). I've posted on the Toyota forums asking those that are running these lights to see what they say for a replacement. It's a 4 month old post, so we'll see if I get an answer.

 

Also, I don't understand why the link provided above in 1st paragraph uses a relay. Can you not just run two signal wires together to a single filament bulb if each wire has diodes?

 

Yes you can as long as the two sources can supply the current required.

 

I accidentally stumbled across a device designed and produced for exactly for what I'm trying to achieve.

 

I'm looking to install some OEM rear fog lights on my wife's new vehicle. I was going to wire them up just as brake lights, and swap the incandescent for LED if possible (working on socket compatibility now).

Where do you live? Is the modification you're contemplating legal?

In the US, rear fog lights might be illegal in some states. When allowed, only the left (driver's) side is wired. If both are wired, drivers might get confused.

 

Also, I don't understand why the link provided above in 1st paragraph uses a relay. Can you not just run two signal wires together to a single filament bulb if each wire has diodes?

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Do You have separate Turn-Signals/Brake-Lights ?,
or do both functions use the same bulb ?
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I've checked my local laws (Florida). As long as they do not flash and are red when facing the rear, additional brake lights appear to be unregulated, similar to a 3rd brake light. No laws requiring them, no laws prohibiting them. I'm not tampering with any factory (required) lighting. Many people in Florida have lights under their car/truck and motorcycle. There are specific laws prohibiting certain colors, facing certain directions, or flashing while stationary or while moving on public roadways.
Yes, I have separate turns and brake. The brake and running/parking tail lights are factory LED. The turns (front and rear) appear to be incandescent. Both functions do not use the same bulb.

 

As far as brake light legalities, these will be additional lights. The factory lights will still function as designed by the factory, to existing law requirements. Some newer vehicles are relocating the two brake lights to the bumper, with a 3rd high-mount as normal, like the Kia Sportage and Chevy Bolt.