Need some electrical help with my ‘68 Plymouth Valiant 100

snailgarden said
Guessing this is the coolant plug for the intake manifold

there is no coolant plug. It is just one of 10 or so intake manifold bolts and if it is broken off in the head the only way to fix it is to remove the broken off piece and putting a new bolt (along with new intake gaskets etc) That might be best left to a shop. It might come out easy (rarely) or REALLY hard and if you mess up the threads in the head you have a bigger issue.

snailgarden said
I then used a circuit/fuse tester with the light on it, touched those fuse ends and none of the three lit up, even with new fuses in them

Some of the fuses are only live when the switch to the device is turned on and if your test light is connected to Plus voltage and the tip is on a Plus voltage it will never light up. so be sure the wire end is attached to a good ground.

is your car an AC car? post a photo of your dash the center part.