It is likely the ignition system. The high frequency of the sparks causes radio waves to be generated in the spark plug wires, the straps that connect the distributor housing to the spark plugs. Your best bet is to replace the original style distributor housing with one that uses the more modern flexible rubber coated cables and then make up a set of cables using resistor wires. These wires have a carbon core that offers some resistance to the spark current. That reduces the radio waves emulating from the ignition system. Because the radio waves carry energy away from the ignition system, the resistor wires actually produce a hotter spark at the spark plugs. Also replace the high tension wire from the coil to the distributor housing with a resistor wire.
Another thing to do is to add a capacitor to the output of the alternator. The alternator can also produce radio waves that are broadcast through the wiring system. An ignition capacitor (condensor) can be hooked to the output terminal on the alternator and the other side grounded with some sort of bracket. Don't tighten the bracket too much or you will damage the condensor.
Other than that, you can continue to save up and buy a wired turn signal system.
If you have the indicating lights outside the car at the rear, try mounting them inside the car at the rear window. They will get a stronger signal there.
