2000 elantra - No 'main' breaklight, but 3rd breaklight OK

I have a 2000 elantra that has no 'main' brake lights. 3rd brake light is
working OK.

Tailights and signals work when headlamps are turned on. Only the main brake
lights don't iluminate. Bulbs & fuse (Tail) look OK. I got a wiring diagram
out of a repair manual, and it shows the 3rd 'eye' (breaklight) wired to the
same bakelight switch hot lead going to the main breal lamps, so if the 3rd
eye is working shouldn't the main breaklights?

I noticed the diagram showed a seperate fuse for l & R legs of the circuit,
but my fuseblock only has a single fuse for brakes, is there another set in
the engine compartment? Could this be battery cable (ground??) related. I
recently replaced the cable with an OEM replacement??

 

They "look" OK, but are they? Sometimes the filaments gets a tiny break
that does not show readily. Invest $1.99 in new bulbs to be sure before you
tear into something else.

 

I'd be surprised to see separate fusing for each side. Have you put a test
light or a voltmeter on the circuit yet? Since the directionals work, the
brake lights should also since they both use the same filament, but I would
be looking hard at grounds back there. Bad grounds cause lots of bizarre
problems. You state that you recently replace the battery ground cable -
why?

 

The battery harness was replaced due to it rotting out at the terminals. On
the Elantra the + & - are in the same harness... I also noted a comment from
someone else about the turn signal using the same filiment as the brakes. On
the 1999 the turn signal has its own bulb... I'll run a multimeter across the
fuse and the bulbs to verify they are good. I don't know if the cable
replacement coincided with the no breaklights (timing) just noticed it, then
thought, 'What has recently changed? I don't suspect the fuse as the 3rd
(center) brake light works fine and is on the same circuit as the 'main brake
lights', but electricity can be a strange animal.

 

OK, the results are in... Yup, there really is a L & R set of fuses. And not
just for the tail lamps, but for the parking lights and headlights.
Interesting. But they weren't the problem. Broke out the trusty multi-meter
and Ohm'd out the fuses (all good) then hit the bulbs (that look just fine),
and bingo - Opens in both bulb's brake filiment. Replaced the bulbs and now
good to go. Go figure, I lost brakelights (both L&R) at the same time, and
spent a bunch of time looking for a problem someplace else.