You will not need to have raced for long to have heard that shouted from the rostrum. Everyone thinks that as soon as their car doesn't do what they think it should that some fiend has switched on their transmitter in the pits. I will say this now and repeat it frequently -----VIRTUALLY ALL RADIO INTERFERENCE COMES FROM YOUR OWN EQUIPMENT---- yes, whatever you may believe, the fault probably arises in your gear.

TRUE OUTSIDE INTERFERENCE

There are instances when someone is mistakenly on your frequency, or heaven forbid really have it in for you, but these show up very easily and are easily dealt with. Remember to check very carefully on the heat and final listings that you have the correct crystal. If you need to check your car is working, or set up a servo or speedo, then YOU MUST GET THE OK FROM RACE CONTROL. Do not just look at the list and assume that the frequency is clear. Ask permission.

Now that's got over the easy bit.

Remember what I wrote VIRTUALLY ALL RADIO INTERFERENCE COMES FROM YOUR OWN EQUIPMENT, and can be very difficult to trace. First a quick science update.

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About everything is wrong in this Tamiya installation, long motor wires, aerial running next to speedo, control wires running everywhere.
All wires or components carrying a changing current emit electro-magnetic waves, and the larger the current and the quicker it changes, the stronger the radiation. Our gear is carrying phenomenal currents, at a level that can cause engineers nightmares. Spikes in an unprotected motor can reach hundreds of amps but these currents can be tamed.
The reciever in your car is working with fairly weak signals from your tranny and filters out the meaningful signals from the general mush in the ether. Anything that increases the background levels makes its job harder. Once safely into the reciever the signal is boosted and is thus less prone to interference. So a good installation will help reduce the mush and increase the signal reaching the reciever. In general it easier to reduce the unwanted radio waves than increase the signal so that's where I will start.
When installing radio gear remember to

1)Try and keep all wires as short as possible. The shorter the wire, the lower the signal it will radiate or the less it will act as an aerial.

2) Keep the reciever as far from the motor and speedo as possible. This fits in with point 1.

3) Do not allow power wires to cross or come near signal carrying wires, especially the aerial. A well known example of this is in 2WD where saddle packs can sit down the centre of the car and the reciever is often found near the battery. If saddle packs are used, the connecting wire needs to be tucked out of the way of the aerial, not left loose. See right.

4) Fit 3 capacitors to the motor, one each from the solder tags(+ve and -ve) to the can and one between the two solder tags. This filters spikes coming from the brushes (the major cause) before they get into the wires. Also fit a 47uH choke in any FET servo power wire. This filters spikes coming from the battery.

There is some debate over whether coiling the servo and speedo control wires is a good thing or bad. My feelings are that a neat coiled signal wire is OK compared to a messy, non-coiled wire. Delving back into my physics, a coiled wire will radiate and recieve radiation along the axis of the coil and not the sides. This should make it less likely to interfere with the aerial or pick-up motor wire radiation if properly positioned.
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Now for some specifics. I have already mentioned the 2WD situation, especially Losi XX and Associated RC10, both of which are made for the reciever  to sit one side of the battery and the speedo on the other.

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Losi XX4 4WD. This uses saddle packs with the aerial just infront of the rear gearbox. Some people have found that having the battery with the connectors at the front, as would seem the best with the shortest power leads, can be a problem because the wire connecting the two halves of the battery has then to run over or just behind the aerial as can be seen right. It is best to put the cells in the other way and have the connecting wire running infront of the reciever.