72 blazer fan speed help!!

[Blazediggity]

Hey guys, long time reader first time poster,

OK, so my 1972 Blazer w/ AC is having a problem with the hi speed on the blower fan turning on, low and medium work, i checked the ground and changed the switch with the same resoult, I tryed swithing out the little 5 prong box (relay i think) with a new one and nothing worked (hi low and med), thinking I got a dud From ACDelco I went back and got another one, same resoult, Is there some thing else that i should check? Any advice is much appreciated...