Br E or Am E? In Br E we don't say 'in back of' (except in printed instructions - "pass tab A through hole in back of <something>" ), and rarely use 'in the back of' (only when 'back' is a noun - "There was a hole in the back of the chair").
I don't know the details, but Am E (with some restrictions???) can use 'in back of' to mean 'behind'.
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