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In this picture you can see a capacitor at the top. Somewhere I saw that it was a radio capacitor. I guess it is noise suppression? Anyway, do I need to use it anymore if buying a modern radio? How do I test it? And then where does it connect to? And does it connect to the + side or - ?The lead has a very thin sliver of metal for I believe to slide into a socket along with the prongs of something else.
Any insight is appreciated.
The capacitors were for ignition noise suppression. Your AIM shows all of them in the U69 section. I don't know whether you still need them if you go to an aftermarket radio.
I am running an aftermarket radio in my 73. I left all the capacitors in place but did not connect them. If you get any whining from you new radio, there is an inexpensive noise suppressor you can get to attach to your alternator, for about 20 bucks.
Actually, there were various capacitors throughout the car for eliminating 'noise' from the output of the radio. The one on the alternator got rid of alternator whine; the one on the ignition coil got rid of ignition pulses; the one at the brake pedal got rid of the voltage spikes when the brake was depressed/released; the one on the HVAC blower got rid of blower motor commutator whine; etc, etc.
The spark plug and wiring shielding also aided in elimination of ignition system electical 'noises'.
And, NO, you don't need it anymore. But, I suggest that you just disconnect one end of it and leave it in place for "posterity".