Usually the antenna plug on aftermarket radios is correct female connecter for American vehicles. I have a Pioneer in my truck right now, had a Dual and some other one once before that, Sony or Alpine I think it was. Never have I needed an adapter to connect the stock antenna cable into the radio. Even when my cable got broken and I pulled the one out of my 73 it still worked perfectly. Have never had reception issues with the windshield antenna.
It sounds like you have some kind of cable problem or the radio could be bad. I accidentally pulled the terminal of my original cable and when I pushed it back on it made the reception go to crap. Yanked the cable from my 73 and works fine ever since. Try getting another cable, maybe a wrecking yard or some place and test to see if it improves. Also make sure the connector at the windshield base is still intact and not broken or pulled away. Possibly try another radio if available to see if signal quality changes. My old dual deck got canned because the screen stopped working and then so did the memory buttons. The signal was bad if I used the buttons but if I tuned to the station it came in clear. Not sure why lol.
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