You may need to make up chokes with separate wire, then patch them into the existing wire.
I agree. Get some toroidal cores and make a common / differential mode filter with the right material mix for the frequency involved and then put some .01's line to line across the device side of the filter. The high series inductance of the filter in conjunction with the bypass caps should nail the problem. Keep the filter as close as possible to the device.
Before you do anything, remove right at the device the AC feed allowing it to run on its internal batteries. That will give you an indication if its coming in the AC feed or the interconnect control wiring. My guess its not the AC, but in the control cable. Still, its prudent to do the test so you partition the problem.
Joe - GMS