As I recall the large yellow wire under the dash is hot when key is on. On
my son's 72, I believe there is a 3 lug connector that you can tap into for
accessories. Don't use the second small post on the starter solenoid. It's
hot only when the solenoid is on. Otherwise, you're reading lower voltage
fed back through the wire after it goes through the coil's ballast
(resistance) wire. The second terminal is used to feed straight 12v to the
coil for a hot spark on start.

"Big Al" > wrote in message
...
>
> ".boB" > wrote in message
> . com...
>> Joe wrote:
>>> I'm installing an MSD-6A in a 1972 Ford Mustang, 351C-4V, manual
>>> 4-speed toploader. I need to connect the small red wire from the 6A to
>>> a +12V switched source (needs to be +12V at the run and start
>>> conditions of the ignition switch). Any recommendations as to where to
>>> tap in for this source? On the starter relay, there one of the small
>>> posts is +12V in the run, but I don't know if it is +12V in the start
>>> position.
>>>
>>> Thanks...
>>>
>> There are a number of places under the dash to find a switched
>> circuit. Start at the ignition switch.
>> Ignition boxes require a fair amount of juice to operate, just take
>> a look at the size of the red power lead. And 30 year old electrical
>> wires are not all that sturdy. Use a 30A relay.
>>
>> --
>> .boB
>
> Doesn't the large red wire go to a circuit that is hot all the time? The
> switched wire is small.
>
> Al
>