Miata taillights - MY simple stupid Miata question

A friend noticed the manic Miata's brake lights were not working properly. This is new in the last month or so.

At first the driver's side brake light was out but at least the CHMSL worked, as well as an auxiliary brake light the PO added on the roll bar.

I pulled the left taillight housing and the brake / taillight bulb was shot - blown out, and glass loose from the socket. I put in a spare 2-contact bulb I had in stock; it's amber so it's likely not the correct filament configuration, though it lights in taillight mode with the headlights on. But things are still weird:

With or without the bulb in question in place, now NONE of the brake lights work, including the CHMSL and the auxiliary brake light, even though they used to. Taillights work, as do turn signals.

I thought it might be the brake pedal switch, BUT the trailer taillights, turn signals, and brake lights all work normally when hooked up.

Can a wrong or missing bulb kill ALL the brake lights? That seems not right. Thanks.

I've never noticed that - but I'm wondering if you have a disconnected ground. You had to unplug the wiring harness to change that bulb (I think, my LEDs don't need changing!) so maybe one of the terminals didn't seat.

I didn't have to unplug the harness; it was just part of the 2-lamp housing I snapped out of the larger taillight assembly. Then it was a usual twist-out lamp socket.

However, grounds are where I'll start looking. Thanks very much.

Maybe a short elsewhere first killed the bulb and after replacing the bulb the short finally killed a fuse or fusible link?

Well, the glass was loose in the metal base, so I assumed loss of vacuum is what killed the bulb, but you could be right.

Well, berk. Changing the lamp didn't fix the brake lights. Not that I expected it would. Hope springs eternal. I hate wiring.

And triple berk.

Replaced the damaged bulb with the correct one (according to NAPA anyway). No joy.

Horn also doesn't work. Research indicates horn and brakelights share a 15a fuse. Found a blown 15a fuse in the ghetto fusebox under the hood (PO rigged the wiring to add lots of increased wattage, 4-lamp low profile headlights, and separate DRLs). Replaced said fuse. No joy.

Went looking for bad grounds as Keith suggested and found one in the trunk near where the trailer pigtail is spliced in. Secured under trunk latch bolt on chassis. No joy.

Not sure where to turn next. 1996 Miata. Tallights and turn signals work fine. No brake lights or horn. Any suggestions?

Duke wrote: ...ghetto fusebox under the hood ...

Good luck with that. It's automotive forensics time.

Keith Tanner wrote:

Duke wrote: ...ghetto fusebox under the hood ...

Good luck with that. It's automotive forensics time.

I honestly need to replace the entire wiring harness on this car, for my own peace of mind if nothing else. And I hate wiring and interior work.

Check to see if brake light switch at pedal is getting voltage. My guess is it is not (cause horn).

Would seem to indicate some under dash wiring is not right. I bet you have a short under there due to a shoddy alarm/stereo/gauge/etc install. Or ex install.

Look for wires that don't look factory to start.