H2ummer... the VSS is vehicle speed sensor. It feeds the speed of the vehicle to the NAV. If you lose the signal for a period of time your Nav will sort of "dead reckon" based on the speed sensor. Also makes your map progression smoother. I think this is the primary purpose. 05 probably come prewired this way but as you know 03 aren't suppose to even have the factory NAV (disproved by Klaus!) so you have to do this jumper.
89Vette ... Since you have an '05 and Nav is a factory option and your connectors are different you are probably prewired. That would be my guess anyway but that is a guess.
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Jonahs
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