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Thanks @R-REG-SR I found the rubber grommet things I will drill a hole in one tomorrow to thread the red wire through

  and yes I have an aftermarket stereo...

All I need to do then I'm guessing is splice the blue wire into the blue wire on my stereo harness then connect up the RCA cables

Oh and any recommendation on where to connect the earth?

It's not actually that hard.

Thanks guys!