78 vette alternator brown wire lost it's switched voltage that  feeds the windings. Can I supply switched 12 volts from another source besides the gauge or does it need to run through the resistor. I think it gets it's power from the gauge or brake idiot lite. I just have to rev engine  it up a little to get a + charge on the voltmeter now.

The brown wire is from the ignition. If you bypass it with direct ignition power when you shut off the engine current will back feed from the charging system to the ignition and the car will not shut off, just keep running. There is a resistor in line to prevent this from happening. Cars without gauges use the alternator/battery light as a warning and as the resistor. It takes very little current to activate the regulator inside the alternator. The ignition powers the resistor until the alternator starts to charge. With a bulb it lights due to the alternator/regulator acting like a ground. When it starts charging the voltage from the regulator creates an equal current on both sides of the wire. With two hots and no ground the light goes out, and the resistor stops working, until the charging system stops or the engine is shut off. So can you bypass the resistor without tearing into the dash? Yes if you use a resistor, or connect a 168 side marker bulb as the resistor. This would also act as the charging system light. It will work until you have time to tear into the dash. Remember the problem could be in the harness connector through the firewall, and not in the dash. Some tracing is in order. The resistors rarely (sometimes) go bad. I have only seen two, but have replaced a few bulbs. This is usually a bad connection.

Thanks Ken, glad I waited for a reply. Planning on pulling dash this winter to rework all the gauges and check wiring and circuit board.   Will go with checking for a break on the engine side of the firewall. If not I'll just use a switched wire and add a resistor.  Fix it right this winter. Any certain size resistor recommended? Very informative reply. Thank you.

Why use a switch? Just connect to the ignition power. That way it just works without needing to remember to turn it on and off. Forget and you get a dead battery. 100 ohm 2 watt resistor should work.

Thanks for reply.  I'll use the resistor. When I said "switched" I meant a source wire controlled by the ignition switch. Yep, there is hillbillies in Indiana. Thanks again..