Toyota 2008 FJ Cruiser Owner's Manual page 198

2-5. Driving information
Selecting trailer ball
Use the correct trailer ball for your application.
Connecting trailer lights
174
Trailer ball load rating
Matches or exceeds the gross
trailer weight rating of the trailer.
Ball diameter
Matches the size of the trailer
coupler. Most
stamped with the required trailer
ball size.
Shank length
Protrudes beyond the bottom of
the lock washer and nut at least 2
threads.
Shank diameter
Matches the ball mount hole
diameter size.
Use the wire harness stored in
the rear end underbody.
couplers are