My current goal is to test wiring integrity in industrial vehicles where wiring insulation has been compromised by poor splicing/tapping practice. Most wiring is PVC double insulated Fig 8 with a presumable Z of around 100 ohms but I do not have a clue of the Z of single insulated wiring bundle in harnesses.
An avalanche type TDR design is probably better for this exercise, as a pulse may work better than a step one for such the short distances involved in the above exercise but I'm leaving that for "plan B"
The nominal impedance is unimportant, since a TDR detects
changes in impedance.
A pulse and a step are equivalent; the distance resolution is determined by the transition time. I have a TDR with a step output that resolves ~1cm impedance variations with a max range of (IIRC) 100m or so.
Assess how any voltage in the car might affect the tester. The TDR mentioned above can be damaged by the residual static charge in a cable, let alone a DC voltage.
I don't know anything about modern car electrics, but consider whether differential impedance is important. The impedance will vary for many reasons, e.g. what other conductors are nearby.