Dear Cameron,
We do not support the Mcintosh Radios.
Although the GROM unit is recongized by the radio, you are seeing yourself the controls do not work. This is a known issue as some of our previous customer have reported to us. We cannot fix this issue as we do not have this same radio here. The reason why it is hard to get to work is because Subaru has different companies manufacturing their head units for them and each works differently.
Which one of our distributors did you purchase this from?