I assume the bulk of the tuner harness is occupied by a serial interface and line level audio. A serial (rs232) receiveer will typically handle about +/- 20 to 25 V without permanent damage. A serial driver will typically drive about +/- 13V (take care not to damage your "homemade" tuner). Line level audio inputs really should be driven no higher than 2-3V max. Line level audio inputs can be sensitive; driving them to 12V could (and for extended periods of time probably will) cause permenent damage. Driving any 12V ac signal to line level inputs will also sound terribly distorted... Rework with fine pitch surface mount IC's (A/D converters for instance) requires some skill and specialized soldering tools...

Cory
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MK2a 160GB

11 Years later, these Mk2a units still rock...