Best way to wash away the winter?

No automatic wash will do an awesome job. Nothing beats washing it by hand when you can. But for the undercarriage you can go to the car wash where you spray it yourself, use an option that's good for tires and bugs, and use that to spray the wheel wells and underside the best you can. Should work okay on all your unpainted plastic trim as well.

Applying a wax by hand will also leave a longer lasting protection than anything from an automatic wash. I use automatics, touchless preferably, just as maintenance cleanings between hand washings and wax.

 

I would also recommend a good hand wash. I wouldn't recommend using any car washes,but of you have to for sure a touchless. After a good wash clay bar the surface to give yourself a clean smooth surface to apply a quality wax or sealer. As far as the under carriage you could use a set of ramps if you have them so you can spray underneath easier.