Usually, when measuring amps with a multimeter you run the meter in circuit but the amp draw from pretty much any electric car system will be massively higher than any multimeter can cope with and you will destroy it.

What you need for high amps is a clamp meter which has a sprung 2 part ring that you clip around the supply wire and it calculates the amps from induction. You can get cheap ones that do AC but for DC you need a decent bit of kit, something like this: https://www.amazon.com/Amprobe-ACDC-.../dp/B0011DC008