fitting a racetronic gs 30, help!?

hey guys, might be being totally thick here but I need to locate the -ve terminal on the ignition coil (98 GTi) for a rev limiter/launch control system? any help would be greatly appreciated. seems even local nissan guys are stumped.
 
I dont think its an rpm feed, i know the bee r units use that but it specifically says the -ve terminal. Im in conversations with the guys who make it so i'll ask.
 
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