How can one convince a father that putting salt on cars is neither beneficial nor harmful, especially since he ignores discussion on this matter?

It is permissible to alert a father to the Bid'ah (innovation) of placing or sprinkling salt or performing the "Sebou" (seventh-day celebration) to ward off the evil eye or as Ruqyah (exorcism/spiritual healing), as these practices are not وردت (mentioned/found) in the Shar' (Islamic law). The advice must be given with politeness and gentleness, avoiding raising one's voice or arguing, while following the approach of Prophet Ibrahim (peace be upon him) in advising his father, such as seeking the help of a book or a mosque Imam.