Audio Systems: 1970 Motorola intermittent radio fade, car buffs, rhythmic cycle

Question
Good morning,
I have a 1970's era Motorola AM/FM/8-track in a vintage 1970 Firebird.  The radio has worked fine for years but now fades in and out (but only when the engine is running).  If the car is parked and the radio is running off the car battery, it works fine, but once the engine is started, the radio will fade out after ~30 seconds, then fade out and in in a rhythmic cycle.  Some car buffs have told me one of the 3 issues is likely:
1. The alternator is on its way out and can't feed the battery, spark plugs and radio.
2. The battery is on its way out, and is hindering the alternator.
3.  An internal component in the radio is wearing/worn out. (I think this is unlikely since it works fine if the engine isn't runing).
Any input would be appreciated, thanks!

Answer
I believe there was no alternator in 1970 only a small generator some people modified this and use altenator
but it needs to additional small coails to protect the radio
do you have them they are small  with one wire and the size of a d battery or smaller