> << if the resistance values are the
> same(ie. the dash gauge is also a 150C gauge like the VDO aftermarket
> ones) then what would stop an enterprising Vanagon owner from tapping
> off that signal as opposed to adding a separate sending unit in the
> coolant line somewhere? >>
> 
> Impedance.

Wha? How does alternating current come into a 12V DC system? The temp
circuit is a thermistor and a gauge on a 12VDC loop.