OK, What year SC? Where was it originally purchased ? You can always ask a dealer to run the vin# and find out every detail on the promotion to see who mfg's the detector. Was the alternator serviced at a Lexus Dealer ? Have you looked to see if there are any loose/disconnected wires ? The more info supplied on this subject the better your chances are to remedy the problem.
The only thing i want to know is where the radar boxes are located??, I changed the alternator myself.
Call a lexus dealer and ask them, They should know. If they run your vin# they will be able to find out about the promotion and therefore find the location of the detector. Your best bet Just want to get you through this ;)