Find the remote lead that goes to the aerial, cut it and fit a small push switch somewhere under the dash or in the centre console.
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Its
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*usually* blue, on an aftermarket stereo.
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Bear in mind if you have an amp, that it will normally use the same wire to use it to remotely switch on
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(so cut after the amp joins it!)
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My high end Pioneer av system does this.
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.
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.
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whereas the previous pioneer headunit
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(a jap one), only extended the aerial once you selected it.
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