OK, reporting back...
I have tried three of them now, that all work.
First, I bought this one, and thought it didn't work -- because the 1/8 end was "too fat" to fit into the WM08 securely. HOWEVER, I just recently realized that the wide chrome/metal part UNSCREWS, and can be removed, revealing a thinner, plastic connector end on the inside, and the thinner plastic portion fits very nicely inside the WM08 -- so actually, this one works great (I tested it for sound and it sounds real good). It was from edit and was only $2.69 shipped, from China...
Audio Aux 6.35mm 1/4" Female to 3.5mm 1/8" Male Stereo Headphone Adapter Cable
The only issue with this one is the male end is STEREO, so it MAY not cut off the Equinox speaker if plugged directly into the unit; I only tested it on the WM08 where it works well.
Anyway, here's the second one I bought, when I THOUGHT the first one wouldn't work, and it also worked well (and it's stereo female to mono male, so it SHOULD work in either the WM08 OR plugged directly into the Equinox...)
edit
This one was $5.99 and free shipping, but it's unavailable right now.
But this one was in my headphones case, that I lost when I lost the case, headphones, and all the contents (long story!)
So, when I lost that one, here's the third one I bought most recently, from ebay, for $9.99 and free shipping (and it's ALSO stereo female to mono male, so it should ALSO work in either the WM08 OR plugged directly into the Equinox...)
D'Addario Planet Waves 1/4" Female Stereo 1/8" (3.5mm) Male Mono adapter
Again, to repeat, all three of these work well, on the WM08. I did not test them on the control box connection itself, to confirm if they shut off the Equinox speaker when plugged in; I suspect the mono male ones WILL; the stereo male one -- the first one listed -- may not; I will check to verify if that is important to anyone.
Obviously, NONE are waterproof.
Hope this helps!
Steve