ACC-3.5-RCA will key the FLEX 6600 with NO Maestro

Jim:

This doesn't answer your question about the accessory cable,  I'm describing my Heli pro 7 set up .

The standard PRO 7 cable comes with a Mono 3.5 mm plug (Tip Sleeve) for the MIC and a 3.5 mm stereo plug (Tip, Ring,Sleeve) for the headphone audio .   I plug the mono plug in to the MIC connector and the stereo plug into the head phone audio.  Both mic and headphone audio work great.  Are you sure that you don't have the two plug interchanged at the rear of the radio?  I am very proficient at plugging the right thing into the wrong place.

The 6400/6600 does not have PTT connection on the MIC jack ( SEE page 27 of the 6400/6600 hardware manual).   I have a foot switch connected to the PTT input ( RCA jack on rear of the radio) to key the radio.  IF you use VOX don't need the PTT input.

I also have a Behringer MIC connected to the Balanced input and use profiles to switch between the headset and the mic.

Hope this helps and you get the radio going. 

Gayle K0FLY