That sucks. (Assuming you are asking for a Raptor 660) From what I remember, the thin black and yellow wire is used with the thin blue and black wire to control the solenoid that allows power to flow from the battery to the starter motor. Check the stickies for the manual. In the manual, all of the wires are displayed with their respective colours.
There are 2 main thick wires, one connects to the battery negative (black thick wire), the other thick red wire goes from the battery to the starter solenoid, then another thick (think red?) wire goes from the other terminal of the starter solenoid to the starter motor. There is also a thin red wire that needs power from the battery positive (the wire that has the “inline” 20A fuse for an aftermarket solenoid). If you have the stock solenoid, the second fuse is used as a spare.
For the stock solenoid connections, if looking from the top (at the starter relay - assuming your plug was cut off), and the 4 connections are at the bottom (check pic in next post) the bottom left is for the red wire - uses the 20A fuse, the top left is for the black and yellow wire (goes from CDI to the starter solenoid (it sends power from Batt positive through the the CDI to the starter solenoid). The top right is for the Blue and black cable, that goes to the starter switch, which, when pushed, grounds the blue and black wire and the bottom right is not used (there is a cross on its picture in the wiring diagram).
The yellow and black wire with the blue and black wire is used to give power to the solenoid when you push the starter and they switch the thick wires allowing current to flow from the battery directly to the starter through the thick wires.