[Arran]

Why re-invent the wheel? What I would do is pick up a stand alone eight track tape deck from a garage sale or second hand shop, one that has a power amp built in, and use it as it was intended. As for how it operates I am not certain, it may steal power from the solenoid circuit, but how it inputs audio into the tape decks amp is another story, I guess it uses the playback head of the eight track deck.
Regards
Arran