a frost plug is a round plug that fits into the block. Its made to "pop" out if the pressure in the block gets to be too much, like if there was water in there and it froze. I believe ford puts the block heaters in a frost plug on these trucks. Its too cold (-1) outside right now or i would go check mine. You will see 4 i think, on each side of the block below the exhaust manifolds. One of em should have a wire running to it for a block heater.