I have an 06 TH with the Concertone. I have the same problem. Inside volume on 1 is way too loud but outside speakers are fine.I have a good friend that owns a high end car audio shop and he told me he could install some sort of volume knob, if I remember correctly. I'm in no hurry and since his shop is downtown, getting my 38' rig to his shop is not an option for me. It's going to be a house call in my case. I'd suggest calling around to some reputable car audio shops and talk to the installers. I'm pretty sure they could do the same or similar. Sounded like a small aftermarket 'knob' installed by the stereo head unit. Would beat the cost of a new unit. Good luck, hope that helps
'Life's too short to drink cheap beer'
06' Sierra Sport F37SP
12' Chevy 4X4 LB CC 1T Dually
B&W Companion 20K