Fancy but very simple
I ordered a very cheap vu meter from China.
12V input i had
As you can see, it has a little button for mode change, but backside, so i soldered wires on it, and i put another button onto the machine case.
There are 2 wires for power and 3 for stereo sign, so its simple, 2 wires together are coming from sound output of c64.