RAV4_Toyota

Hi: I am thinking of replacing my stock 2011, V6, Sport stock stereo (not JBL) with an aftermarket head unit. However, I am confused where at the back of my existing stereo have three plugs and most of the photos that I viewed have two. Does anybody know what the third plug (blue) for and which brand/model of aftermarket head unit that will fit mine without any modification? I don’t really care about if the head unit has GPS.... thanks in advance.