1970 Dodge Dart- No crank or start- Battery draw

I think you have a stuck horn relay caused by bad horn button switch. or maybe "if you have" the lighted "key in" feature (time delay)

What do you have on the car that is aftermarket? Alarm? Radio? ?

HOW TO FIND draw:

Get a couple of different wattage test lamps. I use an old stop/ tail socket and lamp, as well as a lighter wattage store bought probe type test lamp.

Disconnect the battery ground and experiment with test lamps until you get one the right wattage to light up so that you can see it. Hook the lamp from ground to the battery neg post. The more the draw the brighter it will light. If your smallest lamp is barely visible, you can put your multimeter in there in series set to read current.

Now pull fuses one at a time. Be sure everything is off...and don't forget dome/ courtesy/ trunk/ glove box lamps and the cig lighter. Aftermarket radio.

If fuses are all out and it still lights, unhook the main output (black) wire on the alternator. From there we will have to do some more thinking. There isn't all that much "stuff" that is not through the fuse box. Headlights and the ignition switch for example. You can unplug the ignition siwtch connector to isolate that