Here is a way to check how your noise is getting to your radio,
With your radio on and your car running,disconnect the coax from the back of the radio ,now if you are getting noise from the radio that fire in step with the engine(rev the motor to see if it changes with the motor) it is coming in on the power wires.........As has been said in previous posts, filters will take care of this problem (the large filters that resemble a power transformer but with only two wires are the best) that and the aforementioned run separate power wires (with fuse!!!!) direct from the battery.
If by disconnecting the coax your noise goes away you probably have a deeper problem called RFI..............Radio Frequency Interference.Any power source that produces a electrical arc will make Radio Frequency waves be it electrical motors, spark plugs , switching equipment or that power transformer out side of your house arcing because of dust build up or water vapor.
The only way to cure this one is at the source of the interference ,if it is coming from your vehicle it can be done (all though not completely in some cases) just by figuring out what is the source of the interference and what to do it . Some of this has already been posted in this thread.
electric motors....blower fan ,fuel pumps(both in line and in tank)
spark plugs...Non resistor plugs and wires,this includes the distributor and cap
On the motors, bypass with capacitors (non-polarity type)as stated before
On the spark plugs,replace with resistor plugs and wires(I doubt this is part of the problem as standard non-resistor plugs and wire are not sold for auto apps unless for racing) on the distributor I have yet to find a sure fix for this one,replacing the wires usually cured this one.
Also you mentioned a CB antenna that uses induction to transmit (sandwiched plates with the rear window glass between them) toss this antenna and buy you a good magnetic mount with the coil in the middle or close to the top of the antenna,or get a 102" whip and mount it on you bumper. The induction ant is junk!!
This is just some of the stuff that I have learned in the 40 plus years that I have been installing car radios an CB"s.Over the years I have run it to some strange problems such as the one that I traced back to turn signal flasher. With auto electrical systems you never know.
Good Luck!!
Kenny
Lost your job yet? Keep buying foreign