My friend bought a set of subs that had lights in them

the lights tie into a 12v power source

could i just run another wire from his cap?

Hey,

Run a positive wire from your distribution point to a relay, and use your turn-on lead to switch the relay. Run the output from that for the light power. If you already have a decent sized turn-on lead, fuse it and use it. That is assuming you want it on when the amp is on, otherwise use a switch, etc.

I personally avoid lights like that, but that's me