Since you're probably not going to use your AEM7 to switch freight, why not put the ERR board in the first passenger car? A discreet tether from the car to the loco at coupler level, and careful routing of the wires past the flywheel(s) to the centralized motor will get you command operation as desired.
This approach will also circumvent the need to install an antenna in the loco, that will also be in the passenger car. Best of all, you will only need ONE command board / command car for your whole AEM7 fleet! (assuming, of course, that you add a mating tether to each of the locos you want to control in this fashion.) My $.02.
