I was wondering the same and came across this answer on another website:
"There is no relay , it's an internal function of the BCM - body control module
The BCM inadvertent power supply voltage circuit provides battery positive voltage to all of the interior reading lamps, visor vanity mirror lamps, and cargo lamps if equipped. In the event that any of these lamps were to remain illuminated for a period of more than 10 minutes with the ignition switch in the OFF position, the BCM will deactivate the inadvertent power supply voltage circuit to prevent a total battery discharge condition."
https://www.fixya.com/cars/t25027434-inadvertent_power_relay_2004_suburban
