The EVTM does say there is a choke attached to the rear of the radio, but it doesn't list it as shown in any illustration.
I would think that's a suppression coil.
Do you have a problem with ignition noise coming through the speakers?
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.