01 fxdl I just installed an LED tail light and I connect ground, then running light and it's fine but when I connect brake wire the light dims down and when I hit the brake it only brightens up to what the regular running light is. If I disconnect running wire and connect brake on blue connection it's bright as hell so I know the it's supposed to work right. I'm stumped. Please help!! Thanks in advance

 

there are a couple of ways these are made.
on the simple style, there are two separate circuits, one for the running light and uses either a lower lumen led or a dropping resistor. on the brake side, the leds are high output.

now they have some with a chip that controls the functions. you have a single circuit and when the chip is only receiving the running input, applies a set level of current. when the brake input is seen by the chip, it supplies high output current to make the leds bright but could take away from the running. it is really hard to say. it could be a source issue, bad controller if it has one. what brand??

 

It is a cheapo from fleabay. I should have known better and paid for a good one. Like the old saying "You get what you pay for" Thanks

 

Put the leads right to the battery, 1st with run lead then with both the run and stop lead. Does it do the same thing? If so, it's the light, if not, then further troubleshooting is needed.

 

Like stated, it's a chip to lower the voltage on the LEDs.
The running and brake use the same bulbs.
You get what you pay for.

 

that may be the tail light itself, I did a Kuryakyn on my 04 FXDI that I bought here in the classifieds, it was a plug and play, no issues, and I had the HD R/T/B module. Wish I could say i'm not having a problem with my current set up on my 05 FLSTCI