Aircraft ignition leads are shielded in a coaxial cable. To suppress
RFI which would otherwise be radiated off the HV leads, the shield must
be continuous (no breaks), and the shield must be electrically
connected at both ends, plug and mag tower. Might be necessary to
disconnect each lead (one at a time) at the plug end, and then use an
Ohmmeter to make sure you have continuity from each dangling gland nut
to the airframe.

Similarly, the P leads running from the mags to the rear of the mag
switch should be shielded to prevent RFI from being radiated. Their
shields should be unbroken, and grounded to the mag body, and to the
panel very close to where the mag switch is mounted.