Switched power wire??

Hey guys, what's the easiest switched power wire to use for installing a tach? My dash skirt makes it extra fun to get under the dash, so I was wondering if there's one in the dog house..

power to the coil.

Well, there are two. and they make the test light pulsate rather than light up solid. . I wasn't sure if that's a bad thing. But I assume it would be.

The tach needs to go to the negative side of the coil on the wire that goes to the distributor to count the blinks to read the RPM,,,,,,but the other positive side is 6 volts,,,,,,has a ballast resistor in it to cut the 12 volts down from the ignition switch,,,,that  probably red/green wire goes to the starter relay and meets up with the brown boost wire which then goes to the positive side of the coil,,,,,so I don't know that the tach will like only getting 6 volts for its power from the POSITIVE side of the coil,,,,it surely needs the signal from the NEGATIVE side of the coil to read RPMs,,,,,,

Thanks Donivan..  

So would there be 12V before the ballast resistor?  I was looking for what I would recognize as the resistor, but couldn't find one.  There was a small sealed little box hanging off the big (regulator?) box in front of the radiator, would that be the BR?  I guess I just need to get in there with a tester and find a wire...

no,,,,,I think it is just a piece of wire connected in the wire that comes from the ignition switch,,,,,it could be at the starter relay,,,,,it could be at the ignition switch,,,,,,maybe a Ford guy knows where Ford hides it,,,,,,you might follow that wire from the starter relay back towards the ignition switch and see if you see something out of the ordinary,,,,,,that little piece of ballast resistor wire gets hot,,,so they kind of cover it with heat wrap tape or shielding,,,,,so you might be able to find it and grab the 12 volts going into it for your tach......

12 v going ="TO"!! ballast resistor- after resistor-6.8-7.2 (memory) old -haha tack wire goes to the + on coil -not ballast- now heres one for ya too- should the car not start after tack is hooked up?? tack's BAD!! many i have unhooked & found the engine runs fine. Now -also- if the ballast wire is getting "HOT" ?? its due to the key being left "ON!" or ballast feed -12V wire is not hooked up properly! to a "switched" feed.