No way to do it without drilling a hole. As you say, there should have been one pre-drilled from the lower post hole to the control cavity.

So it looks like you'll have to drill a hole. which means you'll need to get the longest drill bit you can find, in order to be able to get the best angle without the drill getting in the way and increasing the angle of the hole unnecessarily. I'd look for a drill around 3/16" to 1/8" (about 2.5-3mm) in diameter, and at least 6" (15cm) long. You might get away with something a bit smaller than that, it all depends on your wire size.

Hopefully you haven't put the post support studs in yet.