A little trick that I learned from an alarm installer for finding upper and lower wall plates and locations for outside boxes (when siding is wood/vinyl/aluminum/concrete board) is to either use a peice of a clothes hanger, or to use heavy t-bar or drop ceiling hanger wire.
Cut it at a sharp angle and put it on the end of your drill (cordless works fine) and use it as a drill bit. I've managed to drill behind baseboard trim and even hardwood floors to get the bit to give me a marker in a downstairs ceiling. Then if fishing from the attic to the basement, I'll also at the same location drill one up into the attic.
I like to use the 1/8" glow rods (similar to Greenlee push rods, but smaller) and with 1 1/4 inch holes drilled from the basement and attic, I can feed the glow rods from the attic to the basement and pull my wires down throught he inside of the finished wall.
A couple drops of caulking and everything is as good as new. And best of all, the customer is usually left trying to figure out where the wires are.
