I thought the sequentials came in with the 1965 model and stayed
through '66 (and the sequencer was still mechanical).

I'm not as familiar with the '64 but a relay near the front fender
(like, inside the passenger compartment, behind the kick panel?) might
be a current-sensing relay that uses the fender-mounted turn signal
indicators as a way of telling you whether there is a burned-out bulb
at the back end.  As Dave Barry might put it, I am not making this up.

Those 65-66 cars (again, maybe the '64) had a "stop lamp relay" in the
*trunk*.  In the 65-66 this relay is in one end of the sequencer
package -- the thing that looks like a submarine sandwich for Darth
Vader.   You're absolutely right that it was a commonplace failure
point.

Anyway.  The original poster has found a relay somewhere that, when
jumpered, gives him brake lamps.  The question is, is that the *right*
relay, or does jumpering it just backfeed electricity along a wire of
quite likely inadequate size?    Time to look in the trunk, I think...

When the correct relay is found, the way to test it is to see if the
brake lamp switch (in the master cylinder?  Hmm.  I'm thinking that the
brake pedal would be a more customary locale -- time to assume the
"Jacques Cousteau goes off the back of the rubber raft" position, with
safety glasses on and flashlight in hand) is indeed supplying voltage
at the right times; if this voltage is making it to the relay; and if
the relay is grounded (crunchy ground connections are problematic on
all older cars, especially these Main Street Electrical Parade
jobbies).

--Joe

I feel your pain, or at least have done so.    (Though  putting the oil
pan back on was the only thing about the Bird that ever actually sent
me and my ice packs to the doctor.     I got a scrip for pain pills and
muscle relaxants and a lecture about how I wasn't a kid anymore and
needed to warm up, stretch, and take breaks rather than working for
extended periods in unusual positions. Words of wisdom for lots of
situations, including clacking away at the computer.)

I've never really torn into the 68-70 circuitry, but the 65-66 has a
single sequencer, and a double-barreled solenoidal relay with an
octopus's worth of wires coming out, whence output goes to one side or
the other.

I am thinking that failure to sequence on one side but not the other
could have only two causes in that system (again, dunno how similar
yours is):

1. One side is not activating the sequencer. Could be the turn signal
switch or anything downstream of it, up to the sequencer.

2.  That solenoidal relay -- it isn't getting input, or one side of it
is nonfunctionoid.

So when your back returns to normal,  see if the non-sequencing side is
getting a signal to the sequencer, and then whether the sequencer
output is getting into -- and out of -- whatever *else* stands between
the sequencer and the actual lights.

If you've replaced these things, check connections, a notorious source
of wiring troubles.

--Joe