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My first though would be to tell you to take a set of jumper cables
and attach one end or the neg cable to the neg post of the battery and
the other end to the frame itself.....  that will give you a GOOD MAIN
GROUND...

If it fires up..

I will bet that the inside of your ground wire off the battery and
attached to the frame by the rear wheel is just pain corroded ...
Been there done that ...ya got a 40 year old car with a cable that is
exposed to the environment ...  Cleaning the ends just will  sometimes
work BUT if the cable is corroded inside as mine was...forget it...

Lots of luck... I guess I may be like you...can turn a wrench but for
the life of me chasing electrons around ain"t easy...

Bob
64 & 72 Ragtops
76 79 & 95 Coupes...

Unless there's something special on the BB cars, I think the battery
negative ground cable goes to one of the starter attachment bolts on C2
cars, so check that connection first.

I don't have my diagrams here with me, but a common cause of the problem
you are seeing is a bad connection at the two firewall connectors on the
outside side of the firewall from the fusebox (ie, in the engine
compartment) where the harnesses are plugged in... one of the wires that
connect there is a heavy gauge red wire that supplies power to the
vehicle harnesses; some folks hard-wire that red wire, bypassing the
connector, but cleaning the connectors should be enough to do the job.

If you use a multimeter or a test light probe, it should be easy to test
from various connections to ground, starting at the battery, then the
red-wire connector, then the fusebox, etc, until you see that power is
absent.

Chuck,

There is no hope for this problem.  It will take a long tedious period of
troubleshooting, probably approaching 6 years or more.

As such, I will offer you a fully running and ready to drive '79 or a '63
coupe that is in the middle of restoration and take that '66 off your hands.

If you don't think either are acceptable, then you can begin the long and
arduous task of fixing it.

When did it occur, before or after you changed the battery?  If only after,
then you can narrow it to a few things if you have a voltmeter (or VOM).
1. Check voltage at battery.
2. Check voltage at the starter solenoid where the battery lead (place
positive lead here) connects and to the starter body (negative lead). If
nothing, check various places with the negative lead such as the engine
valve cover bolts, any ground straps, the car frame, where the battery
negative cable connects to the frame, and finally the battery.
3. Check voltage at alternator. First check positive lead to the red
terminal and negative lead to the battery negative.  If you have voltage,
move the negative lead to the alternator case, the engine, the car frame,
and so on.
4. Check voltage at the fuse block inside under the dash. If nothing, place
the negative lead on the battery negative, using jumper cables if needed for
the distance.

If you consistently get voltage as long as you connect to the battery and
not whenever connecting elsewhere with either lead, then the battery cable
is probably bad, whichever one you cannot get current on.

If you get voltage to several places, like the alternator but not inside to
the fuse block, you could have a blown fusible link. These are designed to
protect the car and are located at the starter and the horn relay.  They are
a short piece of wire connected to the main wire by a splice that looks like
a miniature Tootsie Roll in the wire.  Check the voltage at the terminal and
in the main wire just past the splice.

Good luck.