First, check to make sure your reverse switch sensor is getting a good connection. There will be two leads down near the front sprocket. One will have a plastic elbow (neutral sensor), the other will be a screw through an eyelet (reverse). The reverse sensor switch can get crud around it, giving it a poor connection. Unscrew the reverse switch lead, clean the contacts, and reconnect.
Second, check the wires at the CDI. The wires for the 660 CDI were routed poorly from the factory, giving them a sharp 90 degree bend just before they go into the CDI. A lot of your sensor leads connect back to your CDI here. Pull the CDI out enough to check the wires and make sure they haven't rubbed all the way through. I had this happen to my 660.
Second, check the wires at the CDI. The wires for the 660 CDI were routed poorly from the factory, giving them a sharp 90 degree bend just before they go into the CDI. A lot of your sensor leads connect back to your CDI here. Pull the CDI out enough to check the wires and make sure they haven't rubbed all the way through. I had this happen to my 660.