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put in a new wiring harness. replace every actuator under there. might as well put in the clutch pedals cuz you know you are gonna put a 6 speed in it. gold plate every bracket... JK. but there is no telling what you will need to do. you are talking dash pad off the top? you have to bow it a bit to make it short enough to go through the a- pillars. and you will be scared crapless you are gonna split it right down the middle. and it could happen. might be best to pull the seats and the console and see what you can get to from underneath.
I'm guessing you have a one piece dash like my 78. I'd avoid removing it if you can. To get it out you need to bend up a section at each end, but I struggled with this and I didn't have my screen fitted at the time. In the end one end of my dash broke. If you really have to remove it you might consider cutting the end section off, at least then you'd have a neat break that you could glue back in place afterwards and probably wouldn't notice too much. With the centre console, speedo and rev counter and the glove box all removed you should be able to access most things.
the little cans the vacuum hoses hook to. i was kidding. talking about how "while I am in there" can translate into replacing everything you can find.. my rule is your car is a bit of a selfish little thing. whatever you buy her, she will demand something else. so best to let her tell you what she wants. in the process of finding all the vac lines, you will probably find the disconnected or broke line that is the issue. unless it is the heater control itself. i think willcox has a hvac control test procedure.
I have had my 78 dash out and in a few times. If you remove the speedo pod and the gauge pod/radio you should be able to get to anything that might need attention WITHOUT removing the dash itself. While you are down there, look at the flapper door on the pass side. The rubber usually gets worn and they make replacements fr it. That door will keep the outside air, outside. Once the pods are out you can remove whatever ductwork is there and you will be able to get to everything.
Use a heating pad placed on top of the Original Dash Pad to make it pliable for removing. If you have a restoration dash pad, it will most likely be flexible enough for removing. I would personally have the mechanical speedo professionally repaired, and install a new lubricated cable. Also you should address the electronic tach if the one you have is still using the original pc board.
vacuum doesn't suck that bad. hoses get pulled off. or mice chew on the hoses. or the fitting (if you can call something that small a fitting) that the hose hooks to breaks off the control or servo. we used to fix those vacuum fittings with a hunk of the middle of a ball point pen. (the empty end. not the one full of ink) drill the right size hole. it is plastic after all. drop of crazy glue on the new piece and slip it in the hole. before pulling anything else, pull the console and see if you have vacuum at the hose coming in. if you break the dash pad and find out you just needed to pull the console, you will be inconsolable.
I am doing EVERTHING else on my car before I replace the interior. And when I do, I am fixing EVERTHING while the interior is apart. I have nightmares just thinking about it.
I am in the middle of pulling my dash out as well. I replaced the speakers, headlight switch, all new bulbs for everything (LED). I addressed any wiring harness issues as well.