I’ve figured out most of the Fiero’s electrical issues besides this one. The dash also lights up when the brake is pressed. Why is that?

That usually indicates that there is a bad ground.....probably in the tail light harness.......

What year and model?
High level brake light is fed from different circuit than other brake lights on notchback models, and grounded separately on all models.

[This message has been edited by olejoedad (edited 03-13-2019).]

It's an '87 GT. I had the rear clip off a few months ago, and the ground on the frame rail was fine.

your wires are not correct in the brake switch.
I forget if they are orange or yellow.
start there under the steering wheel.

quote Originally posted by BodymanBill: your wires are not correct in the brake switch. I forget if they are orange or yellow. start there under the steering wheel.

I wonder how they would’ve gotten messed up. I’ll have to look at them.

See my Cave, Sneak Path and Lighbulbs
1 or more Rear red tail/brake lights have iffy sockets even if G400 is perfect they cause a sneak path.
all brake side uses same blue wire as high brake for GT and light high brake when have grounding problems.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Would wrong bulbs be a factor as well? I decided to order a fresh set on 2057 and 1156. The reverse lights and rear blinkers are the only ones that take 1156, right?

No. only BU white take 1156.
All others use 2057 and amber rear turn just doesn't wire dim side.

Read cave pages again. Iffy socket(s) may look "good" but no ground. look at actual bulbs when taillight on should see only 1 side light up.
But 1 or more likely has both side on here.

NAPA sells (Or did last time I bought bulbs) bulk pack 2057 that's cheaper then buying multiple 2 packs at other stores.
GT rear uses 8 and 2 in front and NAPA pack is 10 I think.

Stock is 2057 but many have 1157 or 2357 bulbs but won't cause this if the bulbs are good. Sometime bulbs get old and loose in the base and can break ground wire in the base. Crap LEDs can break causing bad ground in them.

[This message has been edited by theogre (edited 03-14-2019).]