I have another Mezuza question. I have seen times that people of different religions hang symbols of their religion on the front of their cars. Is it correct to hang a Mezuza there?

Talmud (Brochos 24a) and Shulchan Aruch (O.H. 40: 1) prohibit the hanging of Tefilin. Binyan Av (88) maintains that it equally applies to a Mezuza left just hanging. Horav Shlomo Miller’s Shlit’a opinion is that it is also improper and unbecoming to hang a Mezuza in such a position. However, the Rov recommends to place a Tzion with the word “Shivisi” inscribed in it. This term referring to the saying of our Sages, and included at the beginning of the Shulchan Aruch that we must place our consciousness and awareness that Hashem is always present in front of us. It is common for some to place in a respected position the Tefilas Haderech (prayer recited when travelling). However, Horav Shlomo Miller’s Shlit’a opinion is that if small children are used to travel in that car and diapers or similar are changed, it is better that it should remain covered.