The side panel passenger side comes off very easy...there is no screws just pop it out enough to let the cable (seen here in blue) drop down from above as you feed it through. I wont give measurements of cable it is visually obvious how much you need.

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I am sure most of you know how to remove the head unit if not there are guides on here...I used tooth picks on my original head unit but dont need to now with my new click in fascia, but once you remove it just feed your cable down the left side till it pops out the bottom as seen in piccy above.

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Remove your hand brake gator and you will see the cable feed to the lighter, you can unclip the cable to give more working room, you see three wires and it is the blue that is needed so pull a small section away from the other two.

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Now you can feed your cable along the side of the paneling I found it easy just to slightly pull the panel to the side and poke the cable up through and then tuck it all up rather than feeding it along and through...then all you do is clip together your feed onto the existing blue with whatever joining clips you use...clip the cable back in close your side panel sorted !

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Now its up to you what you use it for...but I myself used it for my live feed to my head unit.( disconnecting the red feed from the fiat loom and connecting it to the yellow on my head unit for the memory)
And also connected another joiner with my amp feed on. So I can take the on/off switch I installed out and I dont need to manually switch off thus allowing me to relax worrying if I flatten my battery !