A thousand curses on whomever decided it would be cute to wire up the external MUTE line to the in-car DB-9 harness. UGHHH!!!

Yes, I know it's using one of the serial port signals as "external MUTE", but it already has a separate wire for it on the ISO connector -- having it also on the DB-9 is very very silly.

Time to make a 3-wire patch cord, I guess..