CORRECTIONS - SORRY FOR ANY CONFUSION
I assume that the noise is coming through the speaker of your AM/FM radio and/or the police scanner. This type of noise is generated from high frequencies associated with ignition systems. I noise filter is a good solution.
Basically, there are two types of ways to get rid of noise
1) Capacitive impedance in parallel (from 12 volt to ground) with the device
- This means a capacitor connected from the 12 volt source of the device to ground
A Capacitors look like an open switch to DC voltage and allow current to pass as the frequency gets higher.
2) inductive impedance in series (like a fuse - inline with the power wire) with the device
- This means a inductor connected in-line with the 12 volt source to the
Inductors look like a wire to DC voltage and resist current flow as the freguency gets higher
The best would be a combination of the two. There are many electronic supply houses and catalogs that can supply these types of noise suppressors.
I have attached a drawing that may help.
Feel free to email me direct
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