How to run cable from attic inside an external brick wall?
I would cut out a square the size of a single gang box in the drywall, at your target height and see what's behind.  Depending on what you find you can use a set of drill bit extensions until you hit the top plate, then you can pull the wire back down with the bit as a guide. 

Worst case you find you can't do it and you can stick a low voltage, old work, box in its place and screw in a blank outlet cover.