Bogner makes a cable called the Medusa (sp?) It's has 1/4 inch jacks on one end and the Bogner connecter on the other. I use mine with a Ground Control Pro...works great!
I actually still have one of those from when I owned an XTC many years back. If you would like it I'd get it out to you for $30 incl USPS priority shipping (if you're in the lower 48). P.M if you like and I'll get you my phone number.