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There's an elegant answer to this which avoids splicing into the existing audio cabling. I use a Westlink Bluetooth/FM transmitter which is powered by the lighter socket and sits in the glovebox. Tunes into a spare FM frequency and cordless to my iPhone. I'm not big on stored music but I get uninterrupted Martini in the Morning over the internet and the sound quality is better than you might imagine. The cost of the transmitter is around $ 20.
There's another reason for taking this route. I once asked my local Lexus dealer - what causes the eventual demise of early GS/LS cars, given that the engines and gearboxes are so reliable? They told me that often the cabling insulation becomes brittle and fails, causing an increasing number of shorts, which are expensive to trace and rectify. Admittedly, more of an issue under the bonnet but a good reason to avoid disturbing 15 - 20 year old cabling nonetheless!