Best way to shift automotive logic to Arduino TTL
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A Zener diode drops a constant voltage.

I offer this as a suggestion rather than a full solution.

A Zener diode will drop a fairly constant voltage when a mA or more is flowing through it. With the arrangement shown 9 V in will result in about 4.3 V out. With 6 V in the output will be 1.3 V out. R2 and D2 protect the GPIO in the event of an over-voltage.

You will need to tweak this to suit your purposes and standby current.