Originally Posted by quadrifigolio
Check all tyre pressure's are correct, also get a four wheel alignment check and get all the suspension bushes checked.
As for the Bridgestone's, not the best tyre in the wet but i'm sure they wouldn't affect the car to such a point that it is all over the road.
Also check for play in ball joint's and wheel bearing's.
This would be my first thoughts too - the rear of the car shouldnt be loose even in the wet unless you are pushing it and this clearly sounds like you are getting this problem under normal driving conditions.