I'm just curius how hard it would be to build a small sound to light unit that instead of using sound, using the actual signal from speaker output instead of the sound of the speaker.

 

Not too hard depending on your understanding of electronic circuits and your injunity.

 

Using the speaker signal instead of a microphone is the usual way to make a light organ. So, compared to the usual method, it is no harder.

 

So what changes would need to be made to a typical STL circuit? I don't think it's as simple as feeding the signal into where a microphone would be.

I have a velleman STL circuit, one of their little project kits, and I also have some boards kicking around from light rings for subwoofers. I'll have to dig up the schematic from the velleman unit because I have it somewhere, but I also burned out a transistor on it or something because it stopped working.

 

Yes, it may be as simple as feeding the signal into the mic input.
You may need to match up the signal level and impedance of your source to that of the mic input. Most mics require some amplification so you may be able disable the mic amplifier and jump in after the amplification stage.