My Heath/Zenith Model SH-5412 operates only in test mode, the bulbs (new) won't come on in auto mode. I've followed the troubleshooting guide but the problem persists. I suspect the photocell is bad. Even though the unit is under warranty I doubt the company will honor it without me sending the entire unit back to them, which would be cost-prohibitive. So, whats the part number of the photocell for this unit, and where can I find this part?