A few years ago one of my dad's friends asked me to change the heater core in his Ford Ranger. It was the round body style, somewhere around a '96 or '97 model. The square body Rangers were easy to change as a couple of you guys have already mentioned but the round body style trucks are a b*tch. You have to pull the dash out of the way. I priced the job for $100 knowing it would take a few hours. The guy agreed to it but I could tell that he thought I was asking for too much money.
About half way through the job, with the dash unbolted and laying in the seat, the guy came out to the truck to see how things were going. Once he saw how much stuff had to be removed to get to the heater core he told me that he understood why I priced the job for as much as I did. He knew how the old body style Rangers had the access hole under the dash and he thought that this truck would as well. Once he saw how big of a job it was he was glad that he hired me to do it instead of trying it himself.
When I worked at the salvage yard I took out several heater boxes. Some were a nightmare and some were surprisingly simple. I got pretty good at removing dashes. I never understood why the engineers didn't design an access for the heater core on all vehicles. I guess that just makes too much sense.
Another part that's really easy to get to on a G body but can be near impossible to access on some cars is the heater blower motor. I've took out some blower motors that were ridiculously hard to get out.