Make sure that your car has proper snow tires and chains if necessary. In winter, pretty much all major car rental companies will rent you cars with snow tires. This is especially important as you get closer to the Alps, or other mountain ranges. There are many roads in Europe that will require snow chains. Many rental companies will not supply these unless you are already in a high altitude location. If you are renting this winter, be sure to find out this information.
Even though you have a rental car, make sure to check the windshield washer fluid and anti-freeze levels. If you have ever been in on the Autobahn in winter when the roads are wet, you will be glad you have the ability to clean the glass while driving.
Don’t let the gas lines freeze. Apparently diesel can freeze in temperatures below freezing so in mountain locations, the fuel is sold with a special kind of anti-freeze already mixed in. You can also purchase a special additive that will keep it from freezing and add it yourself. This only applies to diesel cars. If you prefer to add it yourself, there are usually two kinds of diesel fuel available in the mountain locations, so it’s up to you.
Most rental cars will come equipped with a scraper, but it’s something you should double check at the station before you drive off. Make sure you are prepared with plenty of warm clothes, just incase you do need to sit for a while. Like someone used to tell me, “Always be prepared!”