I need some advice on my late husband's 1987 Spider Veloce. The plastic window on the soft top is so badly yellowed I don't feel safe driving the car with the top up. My mechanic tried to clean it with some results, but it's still badly yellowed. He feels that I will need to buy a new top, even though the rest of the top is in excellent condition. Is there any way to clean this, or am I resigned to buying a brand new top?