Start with a "finger in the air" test, something quick and very dirty. You can put 10s of amps into this thing without any real dissipation, so do so. All your power comes from the alternator/battery loop.

Do some basic injection test, with an AC source injecting into the chassis near the alternator and battery connections (EVERYTHING ELSE DISCONNECTED). Put a return near one of your expected sensitive bits. A standard welder will do nicely.

Stick a pair of high impedance headphones on a probe and sniff the chassis. If you hear a lot of buzzing, get more scientific and make measurements with a scope. Then you have the basis to accept or reject the work.

Since everyone in the world apart from you uses the chassis as a return, for better or worse, it wasn't unreasonable to do so here.

Steve