Its hard to calibrate for calculating actual amperes, but you could just measure voltage over some piece of cabling you already have in your car.
So for example, measure voltage drop from alternator to battery, if there is clearly one wire and no other current paths that might take some portion of the load. If you need to get real ampere values instead of just a blind graph of differences, you probably need to get DC-current clamp for reference. I purchased “UNI-T UT210E” and have been quite pleased with it.
Another option, somewhat better/easier, but more expensive, is to buy DC-clamp with voltage output. AC would be easy and cheap, but DC is quite complicated.