Grizz,

Somewhere there is a control relay that applies 120V to the heaters when the SMX control calls for electric heat. It's anyones guess as to where it is. The easy way to figure it out would be to call Marathon. You could also have someone toggle a working toe kick heater while you move around the coach listening for the relay. There's a good chance where one is you'll find the others. CC, for example, puts all of the 120V relays in the same location under a cabinet panel behind the main circuit breaker panel.

Good Luck,