where is the rev counter sensor on my skyjet 125 F5 sport??

ive got this skyjet F5 125 sport bike on a 57plate and for some reason the rev counter doesnt work, i have checked that the earth wire (black) and the live wire (green) both function as they should however i'm trying to locate where the yellow and black wire goes to? i assume it is the one for the sensor but i cant for the life of me figure out exactly where it is? can anyone help?