I have 2 neon lights under my dash. I cut the wires, and twisted them together, making the two lights run off of one cigarette lighter plug in. Im wondering, could I have one more light connected to these?

Im wanting atleast one more neon to go on my sub box.

 

Should work fine - It's essentially the same principle that a Walmart Y-Cig adapter uses. Test the setup with all 3 hookup up, unmounted, by touching the wires together, before you go to the trouble of wiring & mounting the new neon; if you don't see any noticeable dimming, you should be good to go.