Every winter I start hearing water sloshing somewhere near the left back tire. It stops during the summer then as soon as our Oregon rainy season starts the sloshing starts again. Any ideas?

Water in the rear left fender panels of the car
should be a grommet plug under the car
on that side to drain the water out
might be a bit rusty there now

if a sedan a leaky boot seal

I suggest you have a couple of assistants rock the car while you listen for the sloshing and try to discover where it is loudest. Clearly water is being trapped in a box section unable to escape. Most box sections, doors, tailgate, etc. are produced with drain holes and sometimes one or more of these become blocked with wax or body seam sealer during production or road tar or mud during use.

A sliding sun roof usually has four drain tubes taking water into the A and C pillars and sometimes these become displaced and sometimes various parts are fitted with rubber non-return drains that might be blocked with leaves, waste paper and various bric-a-brac. Such areas are the heater air intake, rear quarters, spare wheel well...

Cleaning body drain holes, tubes and drains used to be a routine service item on some cars but sadly this went out of fashion since accountants started buying more cars than anyone else.

Sometimes the slight misalignment of a panel during production can leave a box section with drain holes that lead nowhere.