Cruise Control: Benefits & How It Works

Hello everyone!

I know this question might be annoying, but I'm just not understanding something.
I installed an GRA (Gearbox Ratio Adapter) in my 1995 Audi A6 C4 with a 103 kW TDI engine and manual transmission. Now I'm completely stuck, except for one cable. Where exactly does pin 5 on the lever need to be connected? I've read something about the fuse box 64, but I can't make sense of it. Can someone explain it to me clearly?
My plan mentions something about a gear ratio of 39.
Does it really need to be connected to the fuse box? Or does it?
because pin 39 is already in use on the control unit!

Can anyone help me with that?