Didja' have to cut, bend, fold, spindle, or mutilate the heater box to get the old core out?
I still think it's a blend door actuator problem. Do you hear/see anything moving when you slide the temp selector over from cold to hot and back? If not, the door might be stuck - bad actuator, or there's a vacuum leak somewhere.
The lights being off is likely a bad/missing ground wire, somewhere...