Easy to find out. Unhook the yellow from the relay, and hook up the battery with key off
Unhook both wires from the ballast resistor to prevent feedback in tests
Access the connector of the ignition switch up under the column and see what is hot and what is not with a test lamp
What do you have for diagrams/ shop manual?
Starting at one end of the connector, you should have
No18 Yellow is your start wire and is hot only in "start"
Large no 12 Black is switched accessory power OUT to fuse panel switched buss, and should be hot in "run" or "acc"
No 16 dark blue should ONLY be hot in "run" and is the ignition buss supplying the alternator field, regulator IGN terminal, and ignition, as well as cluster/ gauges/ warning lamp(s)
No 14 Brown is the ballast bypass circuit and is hot ONLY in start, feeds power to the coil + side of the ballast resistor. This is the only ignition source during "crank." This wire is why you need to unhook the ballast for testing
No 12 Red is power coming INTO the switch from the ammeter/ harness splice and should always be hot
Then orange, red, red not part of the problem
If you are getting voltage when you should not DO NOT assume it's the switch. You may be getting a feedback loop from another circuit. To find out, unhook the ignition switch at the connector, "normal" the connections under the hood, and see if the problem returns.
"Usable" diagrams from MyMopar
http://www.mymopar.com/index.php?pid=31
Two pages of diagrams:
http://www.mymopar.com/downloads/1970/70DartA.JPG
http://www.mymopar.com/downloads/1970/70DartB.JPG
70 factory shop manual thanks to AbodyJoe
http://www.abodyjoe.com/pictures/Misc. car info/70 Dart Challenger Serv Man1.pdf