Where does the Rambam say, "Habit and Character are closely interwoven. Habit (as it were) becomes second Nature"?
What you are quoting is an English translation of part of a testament that has been attributed to the Rambam (-some details). It is called שערי המוסר. A more literal translation is "Accustom yourselves to habitual goodness, for character is dependent upon habit, habit becoming as it were second nature."
Hilchot Deot 1:7
How can one train himself to follow these temperaments to the extent that they become a permanent fixture of his [personality]?
He should perform - repeat - and perform a third time - the acts which conform to the standards of the middle road temperaments. He should do this constantly, until these acts are easy for him and do not present any difficulty. Then, these temperaments will become a fixed part of his personality.
In Hilchot Deot 1:2
"With regard to all the traits: a man has some from the beginning of his conception, in accordance with his bodily nature. Some are appropriate to a person's nature and will [therefore] be acquired more easily than other traits. Some traits he does not have from birth. He may have learned them from others, or turned to them on his own. This may have come as a result of his own thoughts, or because he heard that this was a proper trait for him, which he ought to attain. [Therefore,] he accustomed himself to it until it became a part of himself."