What Does An Antenna Do For A Radio Does A Built-in Window Antenna Affect The Radio Reception In An After-market Deck In Your Car?

Does a built-in window antenna affect the radio reception in an after-market deck in your car? - what does an antenna do for a radio

My friend has recently launched a new car and moved to his mp3 real estate market. Since the handover, the radio signal is not strong. He took his audio system type, and said it was the antenna. Is that true? Or just a shit, they install?