Hi NeuroWhat's the difference between this one and the one that @mat200 posted? I have seen some videos where they drill up. I imagine I'll still have to go into the attic and double check if it's within the center block. Drilling up might be easier, but I suspect that because this is a wall that is also an exterior-side wall, it will have insulation.
The bit dudemaar posted ( Klein tools 3/4" ) is a wider bit with a stiffer shaft. It is a really nice bit, but not as flexible. I really like that bit for drilling straight down. The other bit that I posted has more flex and smaller bit thus not as good for pulling more cables ( again do a test hole on some scrap wood before attempting to pull cables in wall ). Home Depot also typically has a Klein one which is similar to that one I posted. Sometimes you can find it in a kit with a "L" shaped tool to help guide the flex bit behind the wall. Definitely worth it if you're doing this and like tools. The 3/4" will handle nails you hit better. Plan to have a powerful drill for this work.