The standard Italian sentence (low, colloquial register) would be
La macchina è mia e la tengo come [cazzo] mi pare a me, va bene?
The car is mine, and I keep it (how I [fucking] like / how [the fuck] I want), is that fine?
As noted elsewhere, cazzo is used as a generic expletive, like fucking in English. It is idiomatic to add it without prepositions after come or other interrogative pronouns, exactly like the fuck is used in English.
Note that another expression that is grammatically incorrect in textbook Italian but common in the low/popular register is mi pare a me: grammar rules would dictate that either mi or a me is used, but not both.
The 'endearing' version cazzuccio is non-standard usage; the author used it for comic effect here. He is mixing registers for comic effect: those sentences pair a fake polite / endearing tone with a couple of very vulgar / impolite expressions to invite her not to bother him. See also cariiisssima madriiisssima earlier, where the same literary device is used. Literally it means my dearest motherest: madrissima is another made-up word, where a superlative ending is added to a noun instead of to an adjective. Repeating the "i" and the "s" is used to represent the fact that they are spoken out more slowly, to add emphasis.