invisible bolts for heater core

My son-in-law is trying to fix my heater core in 1988 Grand Marquis Mercury but he can't find the bolts. Does anyone know the location of them? PLEASE!!!!!!

Click here (http://www.autozone.com/servlet/UiBroker?ForwardPage=az/cds/en_us/0900823d/80/10/39/f5/0900823d801039f5.jsp) and then scroll down to the 1979 to 1988 models. it has step by step and diagrams that you click on to enlarge. Hope it helps you.

I remember spending long hrs. doing my wifes 88 crown vic (basicly the same car) and after loosening everything there was to loosen it still would not come loose only to find that Ford did a mid year change with the mounting of the heater core :They double nutted the studs on the fire wall having another nut under the plastic housing (not visible looking at it ) most radiator shops and dealers know that.Good luck