Medium and Heavy Trucks
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Hi ym name is***** will be glad to try to help. This truck has multiplexed wiring meaning the lighting circuit is controlled by an ECM (computer) and the switches are programmed into the computer module. I will see what i can pull up for your T660 ut in the meantime click these links for some links for the Other models that are similar to give you some information on multiplexed wiring. Also look at your Low Voltage disconnect module as well to see if it has tripped.
Keep me posted.