car feels like it's fish tailing all the time not only on bad wheather what isit

Tires are used but I jus replaced the 2 rear tires, I haven't checked the sway bars I'm no mechanic but it's an idea I can give my repair man, and as far as aftermarket parts I know the car has an aftermarket exhaust system and few parts so maybe the previous owner did do a few mods... also my mechanic said that I have a bad strut and coiler on the rear driver side.... it was hailing yesterday and my car spun side ways after a regular right turn going at like 7 miles an hour

You should start looking more into your rear differential. People weld them to make their car slide, and it works really well. Has it had the issue since you bought it?

To be honest it started when I spun on in the rain and the car was stopped by hitting the curb on the driver side but I've attempted repairs since then... and still