yamaha trim/tilt wiring

Hi everyone !

I just bought me a used yamaha trim unit, to install on my yamaha outboard.

I'm am confused with the wiring, the unit has 4 wires Blue, Green, Black and another thinner black wire. (2xblack)

I read Blue is up, Green is down, and black was ground.

What am i suppose to do with the last wire?

Hope you can help me out.

[boats.net]

Both black wires are ground.