2007 Pontiac G6 Rack and Pinion damage

I have a 2007 pontiac g6 gt (2 dr.). I went to try to trade it in today but they said that something was wrong with the rack and pinion. I knew that something was wrong with it because when you change lanes or hit a bump or the road is kind of unlevel the rear end sways REALLY bad. It has been getting worse. I refuse to drive it if it is raining. It was hit in the drivers rear about 7 months ago and the left rear suspension was replaced. My Dad owns a body shop. He "said" he replaced it, if he did or not is anyones guess. We are not on the best of terms. I should have not taken it to him to begin with but I guess that is water under the bridge now. How much does that cost to fix something like that? I don't know what to do with it anymore! Please help!