OK, after reading your other thread you on the same subject it seems you don't have a single socket tail light. You have a two socket housing. You can disregard everything I wrote above because my answers were based on incomplete information and were wrong for your application.

Before you can get a correct answer you need to provide all pertinent information. The reason you don't see a brighter light with either bulb is because the repop tail lights come with the same bulb in both sockets. They come with a 6 volt #63 bulb or a 12v #67. The way they work is one bulb lights when the tail lights are on. The second lights when you apply the brakes. The problem is the provided bulb is dim. It's only 3 candle power. If you don't have the lights on & hit the brakes you only have one 3cp bulb illuminated. Even with the lights on when you apply the brakes you only have a total of 6cp.

There's a fix for that. Are you still 6 volt? Or have you converted to 12 volt? It makes a difference in what bulbs you use.

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