70 Heater
That's one of the big drawbacks to early cars. You have to take the entire dash out to access the heater box. It really is a
lot of work. First class PITA (pain in the a...).
Last year I had the dubious pleasure (?) of taking the dash out of my wife's 70 Mach 1. I would imagine that a 70 Cougar would be the same hassle. On her car the gas gauge quit. I really had no way to test the sending unit without dropping the full gas tank, so I did a continuity check of the wiring. Good. I wanted to check the gauge itself so I had to get the instrument cluster out. That meant taking the dash completely out. It took me a couple of hours to go through the necessary contortions to get at all those stinking screws! The gauge checked out OK. Damn!

Now I had to drop a full gas tank. Take out the sending unit. Check it. It worked too.

Now what. It turns out that the retaining ring that holds the sending unit to the tank had rusted enough that there was insufficient grounding for the gas tank, so the electric circut was incomplete and the gauge wouldn't work. A cleanup of the pickup and a new retaining ring and I was good to go. All that work for a lousy rusted retaining ring!

I did install the new pickup and sending unit anyway since I had gon this far, but used the original brass float instead of the cheap hollow plastic piece the new sending unit came with.
Now that I think about it, I probably should have replaced the heater core and hoses also on GPs just because I was that far with taking the dash apart. Her Mach is a 21k mile original car, but you have to remember that it is also thirty plus years old!