Screening your device (ie enclosing it in a GROUNDED sealed box) will eliminate a lot of ambient noise floating about... but that is only half the battle...
The other half comes down the wiring into your device. Whatever you are building usually doesn't sit sealed-up in it's own box. It's connected to the outside world. It's THAT external wiring that introduces noise into your device.
That wiring could be your SUPPLY from the vehicle's Battery, inputs from Sensors and Outputs to Motors, Solenoids, relays, Aerials etc.
The SUPPLY is usually the biggest culprit. But if you run Sensors (eg a temperature Sensor) and their cables pass close to areas of high noise generation (ignition circuit or even near high-current carrying supply lines), then you will get noise induced into the wiring. There are ways of minimising such noise. Screen Cables is an obvious one, but also designing LOW IMPEDANCE circuits (that will pick up less noise), and adding filters on all I/O's also may be required. In many instances you might have to do more than one thing in order to make your device noise immune.
For real critical applications I've got an ICOM-7000 Transceiver which I load into an antenna right next to my device and it's cabling/sensors. If I can key say 5W into the Antennas at a spread of frequencies in the HF, VHF and UHF bands without my device dying, I know I'm pretty sure it's bomb-proof. It is an extreme test! You can pick up an ICOM-706-IIIG quite cheaply and it is only capable of 10W anyway, so it's just perfect for generating RF noise (which is the equivallent of what most of the young Radio Hams do anyway) for testing.
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