syrancat,
Sorry, can't help you on the zumo cable.
I've got a Nuvi 750 on my vtx and bought a 12volt power outlet at the local autoparts store and hardwired it to the battery with the power outlet in the toolbag attached to the handlebars. This allows me to use the power cord that came with the Nuvi while the GPS sit in a Ram mount specific to the 750.