So "VSS" ends up being the speedometer input.
What is the speedometer? Is it essentially a frequency meter?
[The speedometer in your vehicle's "modern" instrument cluster here is much differ than the situation in a '65 Mustang.]
So, if it is a freq meter (like), making indications based on some varying freq having a 0-12VDC amplitude, then all you should need is an NPN transistor configured for "common emitter", as depicted in Fig. 1 -- Common emitter - Wikipedia
Further, that'll require a resistor between the Arduino output and the base (2k-5k?), the collector resistor could be 1K or so, and "V+" is your 12V.