The tool that is very helpful for the troubleshooting work is a vacuum hand pump with a gauge and a short piece of vacuum hose.
Any manual you read tells you to check vacuum on the line coming off the top of the motor while the motor is running...before the checkvalve filter is a good spot. That tells you the nominal vacuum for the system. If you get to a point where it is significantly lower, you are on the trail of the leak you are looking for.
This is the diagram for a 70-72. I know the pic is a little hard to read, has been handled a few times before I did the PDF file but I will walk you through it.
For 70-72 at least the default condition for wiper door and headlights is open. Makes sense. If systems fail, you would want lights up and wiper door open. Wasn't true on 68 I don't believe and I don't know if it was corrected in 69 or not. There was a change in 69 mid year.
Start with the vacuum coming off the block (hex symbol). With motor running, see what the vacuum is at that point. You could call that the baseline vacuum level for the system.
First you come to a filter called the checkvalve filter. My car didn't have one when I did this work so I bought one for about 8 bucks. Then a vacuum check valve. Best way to check this is to take it off and blow in it in both directions. One direction air will move smoothly, the other way it should not. If it passes air in both directions, you need to replace it. 17 bucks. Mine was dead. From there the vacuum line splits, one line goes to the vacuum reservoir which lives inside the left fender. The other line goes through the firewall to the vacuum switches under the dash. We will come back to them.
The wiper door and each of the headlights have 2 things in common...an activator relay valve (essentially a vacuum switch) and a vacuum activator which has a diaphram inside which does the work. The relay valves for the headlights are near the headlights. The relay valve for the wiper door on my 71 is actually on the right fender, not on the left side of the car as shown in this diagram. It may have been moved, I don't know. The control line for the relay valve is the vacuum line going into the head of it. System vacuum goes into the middle of 3 ports on the stem, the outside ports go to each side of the activator assembly. You can test the activators by pulling the lines one at a time from either side (with motor off) and pulling a vacuum. One way will open the door/headlight, the other will close it. If the vacuum doesn't hold, the actuator may need to be replaced or there may be a leak on the other side of it. Mine didn't move at all for the wiper door. Replace it (100 bucks I'm afraid) and it worked fine. Check with your friends who took off their wiper doors and put on long high rise hoods. They may have kept the vacuum parts that you can reuse.
Wiper door default mode is open. What can cause it to close is vacuum from the system as long as the wiper door safety valve is depressed (the wipers are parked). WDSV s a small vacuum switch right near the passenger side wiper motor and the vacuum actuator. When the wipers are in the parked position and there's vacuum on the system, the switch will allow the door to close.
One of the leads on the safety switch is supposed to be open...releases vacuum from the switch when the wipers are moving from operating to parked position and back.
If you know logic statements this will help:
What keeps the door open is either the safety valve isn't depressed, OR the manual wiper door switch is pulled OR the wiper switch is on or there is insufficient vacuum in the entire system. What closes it is when you have sufficient vacuum and wipers are off AND wiper safety valve is activated AND manual wiper door plunger is in off position.
If you suspect leaks as you trace through the system, pull a lead, pull a vacuum on one side of the line and see if it holds. You can test the leads that go through the firewall by pulling a vacuum and opening and closing the underdash switches. Essentially you walk through the system...yup vacuum here, yup got it there...until you find a spot where it drops.
The aforementioned wiper door rising up on startup and possible 1 or 2 headlights as well is fairly common, particularly I'm finding on BB cars. One should be able to get several ups and downs on the headlights and the wiper door before releasing all the vacuum from the reservoir. Not so in my case quite yet. I am still looking. You could have a cracked reservoir but it isn't likely. There is a vacuum relay high up under the dash on the drivers side which switches when the headlight switch is turned on. It may need replacing. To reach it you must take off the 6x6 plate at the forward most point of the center console on the drivers side. Isn't fun but you can reach it.
If your vacuum lines look a bit long in the tooth, you may want to replace the entire set. I recommend you replace the lines one at a time (they are color coded) so you don't have to start from scratch.
I hope this helped a little. I have another diagram in the other garage that I will bring in tomorrow and see if that sheds some light. As one person said it, you may have bigger fish to fry, but it is possible to get all the vacuum leaks out of the system. Bottom line is keep going until you find the culprit. Our cars are going on 40 years old and stuff just wears out, cracks and gets dirty.
Holler if you have more questions. Hope this was a help.