1966 VW Beetle Radio Issues

I have installed a Retrosound head unit in my '66 beetle,
with some 6x9 speakers in the rear deck and a pair of 5"
speakers in the side panels. There is a small, 4 channel
amp as well. When the engine is not running, it sounds
amazing, especially in such a small space. As soon as you
start the engine, there is a terrible clicking noise that
comes through the speakers, and matches the engine
speed. This car does not have an alternator, but a generator
with points, and the clicking seems to match that. Is there
any way to wire this to prevent the clicking? Some sort of
interference filter? It is not the head unit, amp or speakers
because again, it sounds perfect with the engine off.