The Yellow/Red stripe in your picture is for the electric start! Not sure why you'd have it connected to the tail light?
I'd suggest you disconnect the new tail light, then attach it's wires to the battery (Red to POS, BLK to NEG) to see if that makes the tail light work. Then, while RED is still attached, also touch YELLOW to POS for brake. If that holds true, then:
TAIL:
RED to BROWN (stock harness)
YELLOW to GR/YELLOW (stock harness)
BLK to GREEN (stock harness)