How do I replace a filament light with and LED on an ATV?

During one of my last outings, my tail/brake light got damaged beyond repair so I purchased an LED assembly and it wasn't until I opened the package I realized there were 3 wires (brake, running and ground).  My ATV is older (a 2000), so it only has 2 wire, as positive and a negative for the older filament style lights.  How do I hook this up so that I get my running light and brake light running properly?

Burf5 years ago
Assuming your ATV's lighting system is typical, the filament bulb would have used the chassis as the ground and the two wires would be for the brake filament and the other for the running light filament.
With your new LED assembly, connect the ground wire to the chassis, the brighter, brake light assembly wire to the brake wire and the dimmer, running light assembly to the running lights wire. You may need to test which is which before completely assembling the new light.
Assuming your tail light had both break and running lights then the system varied the voltage going to the tail light. If the ATV uses a 12 V system then it may have used 8v to10v for the running light and then allowed the full 12v through when you hit the brakes. So you won't be able to use the LED assembly you purchased without rewiring the ATV or creating some additional circuit that will send power to the running side when it detects the running light voltage and when it detects the full voltage it lights the brake side for you. Overall it would be best to find an assembly compatible with your ATV.

What LED assembly did you get? Does it have 2 sections to it? i.e. one section is light as the running light and the other section will turn on when the brake is hit? Or does it keep the lights dim on the running lights then goes full bright when the brake is hit?

What did you actually order ? Does your ATV have brake and tail lights now ?