Help for a Newbie on Alpine

I have an old Alpine deck 3DA-W882 with an Ai-Net connection for controlling a changer.  I would like to attach an MP3 player, like an iPod, using an RCA connection.  Supposedly, your Alpine deck needs "Versatile Link" to connect an auxiliary source that is not an Alpine CD changer (the 3DA-W882 is not "versatile link" enabled).  However, I found this on the Alpine website. 

KCA-121B connected to a non-versatile link ready head unit.

A KCA-121B can connect to a non-versatile link ready unit. The Alpine unit must have an interrupt wire (pink/black) in the power harness. With the versatile link cable connected to an Ai-NET head unit, you can activate the versatile link by switching the interrupt wire to ground (-). <http://www.alpinetechcenter.com>

The KCA-121B is a cable attachment you can buy that gives an RCA connection.  Given this information, how would I go about grounding the interrupt wire?  I have installed this stereo a few times, from one car to another, but the wiring harness was already set up.  (from Toyota to Toyota).  Thus, I don't know how to do the grounding of the wire.  Any help would be greatly appreciated!  Thanks!