If the 706 body is up front, try running the RF coax outside the auto body
under the frame to the rear antenna. Is the interference coming from the 706
cabling or from antenna radiation? A quick test can be made by replacing the
antenna with a load and see if the RFI is still in the speakers. You might
also bypass the speaker wires with some 0.001pf capacitors like you would
with a sound system in your home.
Also, you might try placing one of the K-COM type telephone inline RFI
filters in the speaker wires at the speaker.
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