Thanks to Bill and Sophia I got the sidechain working. I am left with
one minor thing that I can't seem to figure out:
When I set up my plug in in Logic and choose an audio track as
sidechain everything works as expected. However, Logic always reports
that the sidechain is available, no matter if there's actually an
audio track routed to it or not...
So how am I supposed to figure out if the plug in should actually
*use* the sidechain or not? HasInput() and GetInput() are always valid
and I am getting ABLs through the sidechain when I pull the input, so
I guess there must be another way to determine if I am getting actual
audio track data through the sidechain or not...
That sounds like a bug in Logic? If your AU's input is not connected or in use, then the HasInput returns true: