It will not matter if you hook them up backward, they will be out of phase with each other. That is the left speaker and the right speaker. one will move forward and one will move backward.
Before you put the bike back together test the radio have someone turn it on and you hold your hands on each speaker. If they both move forward they are in phase, if one moves forward and one moves backward they are out of phase and will sound funny.
If one or both don't make sound you have the wrong wires. When testing it is the first hit when you put power from the radio.
If you get them wrong when sitting on the bike in the middle it will sound like there is a hole in the sound.
Someone might have changed the speaker wire some where along the way. Oh one more thing, just put 1 finger lightly on each speaker cone, not hard you want the speaker to move. Good Luck
If the front speaker is 8 ohm's and you add a 8 ohm speaker now the load to the amp is 4 ohms which might be OK but if you add a third speaker it will be 2 ohm load. Most amps in radio's won't run at 2 ohm's without distortions in the sound. If the speakers are 4 ohm speakers the load to the radio will be 1 ohm. And will not work.