First of all, don´t panic.This wire is only meant to transfer the harddrive signals to the led. I have got a Zorro4 Powerflyer with the same cable and I accidently ripped it off the card ... still works like a charm :-)
But this pin you are looking for now. Take your A1200 in front of you wide open. On the left is the 2,5" IDE controller, two vertical rows of pins. The one you need is on the right row (side where the floppy is) the 3rd pin starting from the top :-)
Have fun !!!