For those of you that own an 06 Sonata GLS and plan to install an aftermarket stereo, be advised that scosche is sending wire harnesses that have an inline circuit board. the circuit board is covered by a little black plastic cover. The circuit board contains some capacitors and some resistors that will severely degrade the audio quality. You will need to cut all the wires attached to the circuit board and simply go wire for wire, attaching the wires to the corresponding color. This will allow the amp in your head unit to power the speakers. There is a harness that scoshe makes that does not have the in line circuit board but I am not certain of the model number. I cannot claim to have figured this out myself. Another reader of this sight, John, figured this out as we were both having the same problem with poor audio quality. I thought this would be helpful to anyone else attempting an install in the near future. My dash looks great with the aftermarket head unit installed. I encourage anyone else to do the same.
Keith