First up, the vehicle body is a pretty substantial conductor. If you're dropping large voltage across two farthest points on the chassis then you've go problems!

What level of ground noise are we talking about versus the signals involved? Most auto electronics I've seen do not use optos (Nissan did use opto sensors for ignition trigger - guess what fails?). If you've got low level signals, either amplify at the sending end or use differential signalling - this can mean just using a differential input using a ground wire and signal wire from the source unit. Ground noise should be cancelled out.

In most cars I've seen, Your sensor wires do not connect to the chassis except at the ECU (electronic control unit). Some signals run in shielded cables, the shield is only connected at the ECU end.