Here's what I saw when I got under my '73 412 wagon just now, with references to the schematic diagram, too. 412 gurus, please correct me if I'm wrong!
If you've got "factory" VPC air conditioning, there's a red wire that goes to the starter's solenoid terminal (labelled 50, on top) along with the red wire from the starter terminal of the ignition switch (manual trans) or starter interlock safety switch (auto trans). Its purpose is to inhibit the A/C power relay while starting, to turn off the A/C loads and (most importantly) the compressor clutch. This wire goes to one end of the main A/C relay coil, the other end of the coil goes to an ignition switch feed. The low resistance to ground presented by the starter solenoid allows the relay to close while running, and when the ignition switch is in "start", the relay coil sees 12 volts on both ends, and de-energizes.
Also, there are one or two wires going to the fuel injection harness, white, hopefully with a "18" or "18-31" stencilled on the wire. The FI wire connected to ECU pin 18 lets the ECU know you're starting for enrichment purposes, and the second wire powers the cold start valve. If there's only one wire, the cold start valve and ECU pin 18 wire are joined together downstream from the starter. These go to the bottom spade terminal on the starter.
Be careful with the FI wiring, test it out with an ohmmeter (with both ends disconnected) to be sure what you've got. The ECU has minimal reverse voltage protection at its inputs, so be cautious with those white wires. I'm looking at building an external protector box for the ECU to make me sleep better at night.
