The right side also has two positive and two negative wires. The subs in Firebirds are dual voice coil - they have two separate coils of wire around the magnet so they have two of each terminal.
Connecting the wires together and attaching them to some aftermarket speaker is definitely wrong. In effect, you are bridging two channels of a Monsoon amp that isn't designed for bridging. Turn up the volume and watch for the smoke from the right quarter area near the spare. Electronic devices only have so much smoke in them and when you let it out they won't work anymore.
You should either use only one of the two pairs on each side or you should get a proper DVC replacement sub.