I'm in the middle of replacing the heater core, resistor and blower I'm G right now. Having just pulled everything out today I can say that while I don't think you have to pull the whole lower dash out a good compromise is taking the center console out. You should be able to feed the wire into the slider easy enough with the center console out. Maybe even by just popping out the right hand slider cover but I'm thinking the center console will make it much easier and worth the four bolts it takes to remove.

Also, when you feed the cable through the fire wall make sure you seal it with a grommet or something. I was chasing a small coolant leak in the passenger's floorboard that would show up form time to time. Thought it was maybe the heater core or hoses. Turned out be the valve on the fire wall is leaking ever so slightly and coolant would run down the slider cable into the cab and then drip onto the passenger floorboard. You probably know all of this but just take the time seal that hole tight with something.
As for me... my whole dash is out. Upper and lower going to powder coating this week since they were out anyway and because of peer pressure from a certain 280GE owner in Atlanta.