Could anyone tell me where it is? I can't find anything in the fuse box under the hood.

Also, what activates the grid heater? The computer or something else?

 

you need to fill your sig out . each is wired differently. follow the two large wires back from the grid heater, it is the 1 12 " high block between the intake manifold and the intake horn. the two wires will more than likely route around the front of the block at a point near the valve cover. they will lead you to two solenoids that look like universal starter solenoids on the other side of the solenoid there is another large wire connected to the battery. this is not really a wire but a fuse link. each solenoid has two control wires one gos to ground the other to the ecm with out turning the key on remove the small wires to the ecm and temporarily connect a jumper directly from the battery to the post where the ecm wire was connected.you will hear the solenoid snap next with a test light or meter check the voltage on each large post if you do not have voltage from the one from the batt the fuse link is bad. if you do not have voltage on the one going to the heater the solenoid is bad if voltage is ok next go to the heater and find the wire that has the voltage on it and touch the terminal with your finger if it blisters it it is ok if not with your test light check the heater block itself to make sure there is no voltage on it the heaters pull about 75 amps each and i have seen loss of continuity through the special gaskets mostly from idiots replacing the intake horn not knowing these are very special gaskets. if every thing ok do the next solenoid the same way if every thing ok on it it is the ecm. if you do not live where it gets colder than -10*f but a few times each year gut and port the heater and remove the large wiring but leave the control wiring connected to the solenoids.

 

Thanks for all the info. I'll definitely troubleshoot it Saturday. I'll let you know how it turns out.