You didn't say what year your truck is, but if it's like my '06 there are 4 White colored clips underneath the mirror that hold the rear shell (and the blinker lens) on to the mirror assembly. You adjust the mirror all the way in to undo the ones to the outside, then adjust it all the way out to get at the ones to the inside. Once you get that off simply remove the lens from the shell.
I don't know if those are the same, but it's worth taking a look....if there's a little white clip in there just take a flat blade screwdriver and press it into the clip while you're pulling out gently on the rear cover.