In my experience a squeaking rear suspension is usually caused by the shock spring rubbing against something, such as against the shock body. If this is the case, some spray lube on the shock should help. Also look for a rubber bushing that might need a little silicone lube. The noise might also be caused by a section of the rear fairing moving a little when you sit on the seat. Spraying some lube around should help with the noise. Just be sure to keep it away from the drive belt, the brake pads/disc and the tires.
A mechanics stethoscope, such as are sold at auto accessory stores for around $10-$15, should also help locate the source of the squeak.