I second the comment to use a relay to control the fuel pump. 15 amps is a tremendous of amperage to draw through an allready taxed aging wiring harness. To pull that much more amperage through it your just asking for a fire. Your friend got real lucky he didn't have a roasted Firebird. A cutoff switch like either style that 63Dartman ot Player1up mentioned is also a very good item cause you just never know when an accident can happen.
To answer the question about which wire specifically. Get a test light and see what fuses are hot only when the key is in the run position and that can be used to control the relay. The relay should be fed with a seperate wire of at least 12 gauge with a fuse going through the firewall directly to the battery.
I have a high powered stereo in my Cuda and the 6 Ga. power wire I ran for it inside was a perfect place to also draw power for my fuel pump.