Using DB9 RI Pin to receive simple signal

I'm developing a simple DB9 Serial Port bicycle computer. The cadence and the speed was easy, as I had the diagram

But I'm trying to add heartrate into it. I have a stereo cable that hooks up to a little polar receiver. If I hook it up like its cadence (it works). I.E. I put the wire to pin 6 and then have a 4k7 go from 6 to

7, and then hook the ground to 4.

But as 6 and 8 are used, I was thinking of trying to use pin 9 (ring indicator) for heartrate. If I hook up the wire to pin 9, add the ground to 4 and then a 4k7 to 7 as well it doesnt work.

So basically, if I've used pins 6 and 8, how do I get another simple pulse? Java has a ring indicator event (so using that pin would make the software real easy). If I wire the HR to pin 9, the ground to pin

4, and then a 4k7 between 9 and 7, that should work, shouldn't it? The HR wire, btw, is a stereo wire, could that be complicating it?

Thanks