The extension cable runs from the back of the radio to the area behind the passenger side of the dash. That is why the upper dash must be removed. Let me know if you need additional instruction on removal of the upper dash panel.
Yes pulling the top of the dash is the only way to get to the cable.
Remove the end panels by pulling straight out. The left panel is shown, and the right panel is similar
Remove bezel around tauge cluster by tilting steering wheel down, set park brake and move shift lever all the way down. Then pull bezel straight back.
Lift the trim panel up to disconnect the electrical connector (1) to the ambient light sensor on the underside of the trim panel
Pull panel straight back.