AZ Vince

I have a 1969 Firebird with 400CID HEI distributor, Im installing the Holley Sniper EFI and need to find the negative wire on the distributor. I see a 3 wire harness attached to the distributor with an orange, black and brown wire (wire maybe original and discolored) so I need to determine which is the negative wire. I also see a 2 wire harness that appears to have a wire for a tach and something else. Does anyone know what the 3 wires are in the wiring harness and which is nagative?