You already know how AM works using a typical AM only CB Radio, they throw audio onto the power line that goes to the finals - you just have to know of a way to make it bigger to handle the larger, greater capacity the audio amplifier has to add more signal onto the power supply to those Finals.
Many older stereos used transformer coupling - read this early 70's - but later they switched to capacitor coupling - they still had to use a winding or two, but that process made radios more "bassy" in response - which back in the 70's everyone that had a Stereo system - talked about THD as a means to "Brag" about how good their system is. So that usually indicated they used Capacitor coupling versus the older impedance driven transformer winding types that took the amps really low output impedance and raised it up to 8 to as much as 16 ohms - that's the transformer style to meet the speaker impedance and reproduce sound faithfully as a means of matching impedance..
The Crossovers they use nowadays may be the only method left to attain some winding matching that can work.
However, the Stereos power supply usually ran about 36 to as high as 50 volts for the capacitive coupling to overcomes the losses in their matching networks to drive the speakers.
So if you could take a typical MOSFET and placed its Drain to Source - directly to the Stereos power supply and feed that extra voltage from the Stereo directly to it, I can see where it is possible to use the Stereos' power supply to at least drive a MOSFET - it won't give you thousand of watts - but it will drive the MOSFET you just have to put the Modulation Transformer in it somehow with enough winds to make it work to take the Stereos capacitive coup0ling network and the Stereos power feed from its' supply - tie it together and you can make it work.
People would say it's not easily done, and no it needs a lot of experimentation and work plus new winding design to inject audio to apply the signal power to the Power supply feed. They did something similar to this when Light bulbs from Edison came out - done to stages of theaters to allow for dimming - and raising of the lights - a coil and core combination called a RELUCTOR was used - it was a basic laminated core with a pivot that allowed the user to move the core in and out of a bank of coils - to handle several hundred watts of power without generating heat or having to somehow provide a dropping resistor - the bank of bulbs used the inductance of the cores as it went in and out of their coils to act as a means to offset power by impedance method - change the impedance affected the lights ability to drop current - hence the name Reluctor.
Nowadays, they use Thrysistor dimmer circuits which are safer and solid state - but the effort you're looking into is a similar process only at the audio level - so you'd have to apply modulation transformer and design one to meet the needs - it's not impossible - just not easy...
