Content provided by: Bob KW4CQ
In order to make my Heil Pro 7 (Electret element) work with my FLEX-6600M, I had to order a Hosa YMM261 Stereo Breakout Cable ($5.00) from Amazon.
Just plug the mono plug into the Black plug on the Hosa adapter cable and leave the Red plug unused. Plug the TRS plug from the Pro 7 into the Headphones jack on the back of the Flex. Plug the TRS plug on the cable adapter into the MIC jack on the back of the radio. Solves the problem beautifully.