To give a formal answer to this question -- As you can see from comments above, there is disagreement amongst native speakers on this matter:
For many speakers of AmE, in back of and behind are used fairly interchangeably as prepositions.
In general, behind represents the relative position to the speaker. So, if the speaker is standing to one side of a car, the object may be behind the car if it is on the far side of the car. In other words, it is not necessary for the relative position to be the rear of the object, but that the object be located between the speaker and the subject.
In back of can represent this same relationship.
The confusing part: if the object has a back or a rear, the relative position does not necessarily dictate the meaning of the preposition. In other words, if the speaker and the subject are both at the rear of a house, they are both in back of or behind the house.
If the subject is front of the house, and the speaker is behind the house, few would now say the subject is behind the house even if it is, relative to the speaker. This will seem confusing compared to the situation with the car above. My instinct is that this has more to do with perception than anything else. The rear of a house is a very distinctive location. The rear of a car is at best a few feet from its side.
If there is any difference between the two: to my ear, in back of sounds vaguely colloquial. I cannot find any data on this, and I doubt this is universal for all speakers of AmE. An attempt to plot an NGRAM of in back of yields relatively few results, especially when compared to behind.