Distortion in Car Radio.

I have Suzuki Khyber in which old Pioneer cassette player is installed. I installed that player 7 years ago. It was a good one with very nice sound.
Now as the cassettes are becoming obsolete and we have a good range of FM radio stations, I use to listen the radio only. But for 2 or 3 months my car's radio is having too much noise due to distortion in radio signals.
When the car is stationary there is no distortion but when it starts moving, it has a lot of distortion. Similarly if the car is stationary but another car passes it, then again it shows distortion.
I went to Saddar and showed it to the person who repair car audio electronics. He said that there is a problem in its channel and he would install new channel. Hence he removed the cassette player and asked me to come the other day.
The next day, he installed the player and said that everything is fine now. As soon as I started driving, I noticed that the distortion is still there.
I came back to that person, he changed some wires which were connected to the grip of player and said that now it is fine. I drove and found out that the problem was still there. Came back to him and he installed a "Choke" and said that it would be fine now. But the problem is still there.
Now my car's player is with him. He was saying that he will check it thoroughly.The mechanic has checked the antenna and according to him it is ok. According to me it is some kind of an electrical interference.
But how to check that?
Does anyone of you faced a problem like this before?