Custom LED third brake light

Years ago(1991), I built a custom LED brake light using 60 LED's that each had a resistor soldered to them (they were surplus).  I then hand wired them in parallel to the brakelight wire and hand drilled a plexiglass strip to align them all and fiberglassed them onto a custom fin.  They work great but the problem is they were designed for a 24Volt fire alarm panel.  Of course the resistors still allowed them to work with 12V, but they weren't as bright as they could be. About 2 years ago I replaced the clear lens with a red plexiglass lens that matched the tailight colour better, unfortuneately they are now too dim to see during the day.  I have an idea on how to boost the voltage to them with a second gel battery and some diodes, but can you see any way to charge the second battery (in the car) when not in use(engine on)? I would greatly appreciate some insight, thanks.  James

P.S. changing the resistors is out of the question, they're buried beneath the fiberglass. :-)

Have you thought of putting in a battery isolator in the engine to charge the batteries evenly ? Just a thought , if this is what you are asking for.

You can get some here ( and many other places ) : https://www.4wdworld.com.au/products/dualbat/

Installation here : https://www.surepower.com/pdfs/180012n.pdf

Thank you very much for the great ideas. I like the idea of using a series/parallel setup with a relay.  I think I will use a double throw relay I have kicking around so that there is less chance of one relay not working. One quick question aobut this setup, when the relay is energized and 24 volts  flows through the LED's it is referenced to ground, is there any need for a diode between load and ground? Or will any other 12 volt component be affected considering they all reference to a common chassis ground?

I made a quick schematic in paint, but I couldn't paste it in here, sorry.

I didn't make tail lights, just a high mount brake light that was moulded to my rear fin.  When I did this, there were only a few hondas and subarus that had them from factory so buying one seemed out of the question. Now they're everywhere!  The plexiglass box took no time to build, just a very flat elongated box in about an hour(overnight drying for the plexiweld). The front, leading edge has holes drilled side by side on the drill press for mounting the LED's into. I didn't even have to glue them because they are naturally tapered and fit snug and straight.(Something I noticed with some factory LED brakelights is some of the LED's are crooked and look terrible.) The wiring took about an hour, all soldered.  The fiberglass work was another story, because I had never done it before, probably a couple of days worth of laying it out and then lots of sanding!!(Do it right the first time and there is a LOT less sanding involved) Looks pretty good, turned a lot of heads back then.

Now people ask me where I bought it.....GRRR Long story ensues........

Just as a sidenote. I have since built some interesting LED strips and clusters for accent lighting. Orange and red clusters mounted into plexi for underdash and underseat light. You wouldn't believe how much light they throw! I have a long strip of alternating red and blue(actually red,blur,blue,red... superbright LED's mounted in a plexi strip similar  to my brakelight that lights my amp. I wish that strip was my brakelight, it actually looks like a red strip with a purple halo effect, similar to "blue dots" for tailights. I also wanted to make some custom LED fender badges but running power to them would be a pain. Someday I would like to build an LED Du meter into the console just not sure how they work yet.