Changed the struts on my 2004 passat and the back end still fishtails when hitting bumps! And when the roads are wet the back end slides! So if the struts were changed what could be causing this?

Have your mechanic check the rear anti-sway bar, as well as the wheel alignment.
The wheel alignment should have been adjusted at the time of the strut installation, but maybe it wasn’t.

Also…what can you tell us about the age and the condition of your tires?

+1 to VDC’s comments.

Also, I’d suggest checking out your chassis, braking and steering systems. A dragging brake, even a parking brake, or any of a number of steering or chassis conditions such as alignment or bad control arm bushings could cause instability under the conditions you describe. I’d recommend having it totally checked out by a reputable shop. Having the vehicle suspended on a rack with the wheels unloaded might be the best way to find bad chassis parts.

I would agree with @VDCdriver on control arms. I guess I’m wondering tire condition also with the wet slide issue.

Fishtails when hitting bumps… Hmmm, Control arm bushings or rear wheel bearings

Slides in the wet? Tires, tires, tires. Are they original to the car? (I’ll bet Yes) Have they ever bee rotated? (I’ll bet No) If they are original, they need to be replaced. I don’t care if they still have “good” tread. The rubber is as hard as a rock and they will not grip in the wet. Replace the rears and start rotating your tires every oil change.

My vote is with the others on rear end suspensions issues and/or aged tires with hardened rubber.