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 in the comments and in the other answer, *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 as a literary device for comic effect here. He is mixing registers for comic effect: those sentences pair a fake exaggeratedly 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 fake polite tone 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. The supernumerary repeating *i*'s and  *s*'s are used to show in writing that these words are spoken out slowly, to add emphasis.

Overall this is a very difficult text to read for a non-native speaker. The author uses literary devices, made-up or uncommon words, expressions from the youth slang and Roman dialect, popular culture references.