Good Method to Add Wiring For Add-Ons

Did you chk mymopar.com ? They have free downloads. Just make sure its factory oem schematics.
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It’s the wire where my probe is.
I’m looking for a wiring diagram. I see Classic has one, but it’s not in stock yet. ML13063A - 1974 Dodge Dart / Plymouth Duster 8-1/2
i just found and ordered this: https://www.amazon.com/dp/B07J1P339S/?tag=fabo03-20