12V feed wanted

[Hawk]

I have an intermittent fault with the 12v feed that goes to the alternator and I need to find a new 12v (ignition switched) feed. 

Anyone know where I can find one in the engine bay of a 79 Spyder?  As a temporary measure I have taken a fused feed direct from the battery but as this is a constant draw I have to disconnect every time I park up!!

For the existing feed, there is continuty back to the connector block and then the wires disappear into the car and I can't trace them (without removing the dash etc which I really CBA to do). 

[Thotos]

Thin-wire 12V switched feed to alternator should be via the alternator charging light in the dashboard. When you loose the 12V feed is the charging light off? If so, change the little bulb and make sure its holder has a good connection to the cluster.

[HFStuart]

The wire going to the fuel cut off solenoid on the back of the carb is ignition switched - you could slpice into that.

[tnsmttg]

Thotos is correct. In simple terms. The cable that you are enquiring about is the ignition warning lamp cable. When the engine is not running and with the ignition off a negative is on the terminal at the alternator. When you turn on the ignition a positive is fed to the ignition warning light bulb and as the other side of the bulb is connected to the alternator, hey presto the bulb lights when the engine is started the positive feed which passes through the resistance of the bulb excites the alternator regulator and the alternator will start to charge. a positive then exists on the alternator terminal so the bulb goes out, as it now has a positive on both sides of the bulb. However, you should understand that due to the resistance of the bulb the voltage at the alternator terminal is considerably lower than 12 volts. The alternator must not be fed with 12 volts. The voltage fed to the alternator via the bulb is an exciter feed. Should the bulb fail or a high resistance be experienced in the bulb or cabling, then the alternator will not charge correctly. Most modern cars now have a resistor in circuit to allow the alternator to charge should the bulb fail. So please don't feed the alternator with 12 volts but investigate the fault and as a temporary measure supply another ignition feed to the alternator but with a bulb in circuit.

[Hawk]

Why can the alternator not be fed with 12v? 

The issue I have is intermittent and I have had no joy in tracing it.  One minute everything works fine (dash light, appropriate charging etc.) and the next I have nothing.  Then the dash light will come in but the alternator is still charging the battery.