If you trace those wires, they'll probably meet up with the stock Nissan harness/plug. The wires you're seeing are probably from an old head unit.
If it's an R33, there is a good guide in the DIY section which describes which colour is which on the Nissan harness.
Lastly... if it's not a power wire (check with a multimeter), it shouldn't do any damage at all. Reversing positive and negative on a speaker generally doesn't do anything harmful either. I would definately recommend testing the wires on their own though.
Get a multimeter, set it to its continuity buzzer, alligator clamp one wire, and touch the speaker-end of the wire with the other end of the multimeter. It'll buzz if there is a "circuit" or "loop".