Check engine light on, code P0677

My check engine light came on about six months ago and I’m getting around to fixing and clearing the codes right before I go to smog in about a month. Fixed all the codes except for this dang P0677 code that remains which says my #7 glow plug circuit is open. I’ve run the gambit of changing not only the #7 glow plug twice with new plugs and swapping it with an original plug that was in the #6 spot but also changing out the wiring harness as well as replacing the GPCM. The code still remains and check engine light comes back with the same code the instant after clearing it. Engine turns over just fine. No obvious mechanical problems. Need to get this solved before smog check time or else.