That sticker you see is the unit/part connecting the speaker to the door. You can easily swap a non stock 4" speakers into that same unit. The speaker didn't look stock/similar to rear speakers taken out of my Sc, although I could be wrong. My blown rear speaker in the thread below.
http://us.lexusownersclub.com/forums...-sound-system/

 

The only way to get the coil that hot is for it to be completely frozen in the magnet. I've not seen that happen before on a stock setup. The OEM amp technically only JUST has enough wattage to do it under very special circumstances..... which you've somehow managed.