לתלות מזוזה במכונית לשמירה-Hanging a Mezuzah in a Car For Protection

(4) It is a common custom to write [God's name,] Shaddai,1This name serves as an acronym for the Hebrew words, שומר דלתות ישראל, "Guardian of the gates of Israel" (Mishnat Chassidim). on the outside of a mezuzah opposite the empty space left between the two passages.2The Kessef Mishneh cites the Zohar (Vol. III, 266a) which states that Shaddai should be written opposite the word, V'hayah. There is no difficulty in this, since [the addition is made] outside.3See also the Ramah (Yoreh De'ah 288:15) who states that it is also common to write the letters, כוזו במוכסז כוזו, opposite the words, י-ה-ו-ה א-להנו י-ה-ו-ה, on the outer side of a mezuzah. These letters are the letters which follow the letters in those names of God - i.e., the כ follows the י, the ז follows the ו.
Those, however, who write the names of angels, other sacred names, verses, or forms, on the inside [of a mezuzah] are among those who do not have a portion in the world to come.4In Hilchot Avodat Kochavim 11:12, the Rambam states:
A person who whispers an incantation over a wound and then recites a verse from the Torah, who recites a verse over a child so that he will not become scared, or who places a Torah scroll or tefillin over a baby so that it will sleep, is considered to be a soothsayer or one who casts spells. Furthermore, such people are included among those who deny the Torah, because they relate to the words of the Torah as if they are cures for the body, when, in fact, they are cures for the soul, as [Proverbs 3:22] states: "And they shall be life for your soul."
The inclusion of these people among "those who do not have a portion in the world to come" is based on Hilchot Teshuvah 3:8, which makes such a statement about "those who deny the Torah." Not only do these fools nullify the mitzvah,5As stated in the previous halachah, any addition made on the inside of the mezuzah invalidates it. (Significantly, the Shulchan Aruch 288:15 states that it is forbidden to add to the inside of the mezuzah, but does not explicitly say that the mezuzah becomes invalidated.) but furthermore, they make from a great mitzvah [which reflects] the unity of the name of the Holy One, blessed be He, the love of Him, and the service of Him,6These are the subjects discussed in the two passages written within the mezuzah. a talisman for their own benefit. They, in their foolish conception, think that this will help them regarding the vanities of the world.7The Rambam's statements have aroused questions from many commentaries who note that, in several places, the Talmud associates a mezuzah with Divine protection - e.g., Menachot 33b, the Jerusalem Talmud, Pe'ah 1:1.
The Kessef Mishneh resolves this difficulty explaining that, although a mezuzah affords Divine protection, that protection comes, in and of itself, in reward for the fulfillment of the mitzvah. There is no need for any additions on man's part. Indeed, a person who makes additions to the mezuzah in an attempt to increase its influence demonstrates that he is concerned with "his own benefit" and "the vanities of the world" and not with the fulfillment of God's mitzvah. Therefore, he deserves the Rambam's severe words of criticism. See Likkutei Sichot, Vol. 19.1. Our translation follows the standard text of the Mishneh Torah. The Kessef Mishneh mentions another version which states "at the beginning or the end of the line." That version parallels the apparent source for the halachah, Menachot 31b.2. This custom is also recorded by Rav Yitzchak Alfasi, and earlier, in the halachot of Rav Yehudai Gaon. It is not clear when this custom was begun.3. As mentioned in Halachah 2, a space is left at the beginning of the line so that the passage, V'hayah im shamo'a, will be written in the s'tumah form.