hello

first off, ill say im not completely serious about neons, or car audio,
or anything like this.

i have a completely crap standard tape/radio unit that probably came
with the car from day one (its a 1984 corolla).

i also just added neons to it the other day, for lols and whatnot.
being the geeks we are in this flat, after we were done installing
them, we thought a brilliant thought - how would we go about making the
neons flicker to music?

yea, the stereo is hardly loud enough to play outside the car, but its
more of a proof of concept thing than anything anyway. and i might get
a better audio system eventually too.

my first thought was to wire them to a transistor connected to the
audio output, but this might actually cause the lights to just stay on
until the stereo is turned off.

is there any way to introduce a threshold current to this, and if so,
what kind of electronic component would i need? and if possible, make
it fine tunable?

A voltage divider (with one of the R's being a pot for adjustment) and a
BJT would do the trick. Throw in a capacitor on the base to make it stay
on for a little bit when it reaches threshold. You could make the circuit
balanced so that it fired on each half of the waveform (+V and -V), but
this shouldn't be necessary. You might want to put a pair of diodes in
the front to prevent any signal getting back to the audio (due to the
capacitor).