I got asked about interference on a E type Jaguar radio the other week,and it got me thinking! (it happens sometimes) you don't have many problems like that anymore so what's changed.This is quite a subject with more and more electronics in cars, mobile phones/ hands free car kits/ GPS systems/ CD players/ DVD players/ high output amplifiers.So where is all the crackly,hiss.whine gone. Well! lets start with EMC (electron magetic compatibly), rules are there for equipment manufacturers to comply with interference rejection, or suitability with other electronic products. My van, Ford connect 2002 don't have a problem with any interference, OK so its a diesel which helps, but there still lots of electrical systems. I have bluetooth GPRS, blue tooth mobile phone Nokia 6600, blue tooth hands free kit + radio FM&MW cd player+Yaseu FT718ND amateur radio  transceiver. Also 12v to 250v inverter.  OK, so its mainly classic cars ones with with contact breakers, noisy alternators/ dynamos or HT leads made of solid wire conductors. So back to basics, first try to correct any fault that might be in the electrics, ie; rotor arm to distributor contacts excessive/ high resistance in HT leads/ contact breakers worn/ alternator dynamo brushes worn/ wiper motor/ instruments stabiliser (behind dash) noisy/ high resistance on earth wiring. Make sure aerial as good earth. This is important as the aerial is a 1/4 wave Marconi, which means that the transmitted signal to the aerial as to see a 1/2 wave length, this is done by a good vehicle body earth to the aerial, think of the aerial being reflected by the vehicle bodywork (like the reflections in a still pond) it then makes it twice the length. Also note that if you reduce the length (telescopic type) reduce it by the top down, the lower thicker part of the aerial as the capacitance of the mast and will  weaken the signal if you reduce it. Note earth lead should be braided strap ie; wide and flat, this is because you are earthing our RF (radio  frequency) and it as a skin affect (travels down the outside of the wire).If its a crackly which increases with revs probably ignition, if its a wine increasing with revs probably alternator/dynamo. Look at suppressors as a RF short to ground, ie; a black hole, and a RF choke as a small tunnel which high RF spikes find it hard to get through. A filter comprises both suppressor and choke and can be fitted at the 12v supply to the radio etc. A new radio should include radio interference rejection circuity and built to EMC standards. HT leads should be suppress usually 5K/10K per meter length, this suppresses the current flowing and not the voltage at the plugs ends. Remember each HT lead as a different length so radiation of RF is at difference frequencies so leads like transmitting aerials, so suppressions is important. Fit 3uf capacitor to main alternator terminal and then to earth. Fit capacitor 1uf to ignition coil 12v supply terminal (usually + sign or sw) then to earth. Earth out bonnet of car with braided strap. Every little helps....................