Here is how I installed my hardwire kit for my dashcam. The car is a 1992 USDM Miata, your car might differ slightly.

I am using a Viofo A119 v3, with the Viofo 3-wire hardwire kit.

It uses constant 12v, key on switched 12v and a ground to achieve parking mode when the car is off.

BTW: Using the fuse taps will make the fuse cover not fit anymore.

Supplies

A dashcam (Viofo A119 v3 is pretty good)
Hardwire kit (might contain some of the items I'm listing now)
1 or 2 ATO/ATC/APR/ATS/Regular fuse taps depending if you're running a 2- or 3-wire kit
Crimping tool for insulated electrical connectors - anything to crimp those crimps properly
Wire stripper, cutter or knife to prepare the hardwire cable for crimping
Rotating hole punch, drill, dremel - anything to make a small cut in the interior windshield trim
Philips PH2 and Torx 40 screwdriver/bits
Zip ties
Pencil to mark the spot to cut

Step 1: Place Your Dashcam in Suitable Place

I placed mine above the tinted part of my windshield for extra stealth, and behind the rear view mirror for a cleaner look inside the car. Placing it now will help you put the hole in the right spot in the trim later.

Step 2: Remove the Rear View Mirror, Sun Visors and Latch Strikers

Windshield mounted rear view mirror cars can skip removing the rear view mirror, obviously.

The rear view mirror is attached with two Philips PH2 screws on NA6. You will first need to remove the black trim that goes around the yellow part in the picture. You can also tug on the whole mirror assembly the mirror and trim will pop off, by virtue of the metal spring you see in the picture.

The sun visors are attached with Philips PH2 screws. Unscrew and put away.

The top latch strikers use Torx 40 bolts. Unscrew them and put away, maybe remember which side they belong to.

Step 3: Remove the Center Windshield Trim

Before you yeet the trim, plug in the cable to the dashcam and try to see where the cable should go into the trim. Mark that spot with a pencil or something.

The clips marked red are attached to the metal frame, and the green are attached to the A-pillar trims.

I was able to pull off the center trim sitting in my drivers seat, with my hands. I started with the clip marked in green near where the striker plate was, you might be able to see it by pulling a little on the edge of the trim. Do a relatively quick pull while holding the A-pillar trim attached to it to make the clip release. The rest of the pulling will be easy because you will be able to put your hand to the next clip and push it off. The clips are made of metal, so they might deform a bit. I'll get back to that.

Remember when you get to the passenger A-pillar trim, the center trim might seem stuck, but there's a clip there, so a quick push will release the trim.

Step 4: Remove the Driver Side A-pillar Trim

This should be simple, do the same pushing with your hand between the trim and the metal frame. The last portion (marked by arrow) is held in by a plastic piece that goes into the metal, it simply slides up and out.

Step 5: Make a Tiny Cut in Your Center Windshield Trim

Use your preferred tool, I happened to have a hole punch begging to be used.

The cut doesn't really need to be that big, because there is some clearance between the windshield and the trim in the first place.

Step 6: Route the Cable

Start between the dashboard and the door.

The plastic trim is flexible enough to simply push the cable in there with some fiddling.

Step 7: Connect the Cables

Protip: the back of the fuse cover has a fuse puller.

The fusebox is placed by the pedals in the footwell, to the left. Remove knee panel by removing two Philips PH2 screws to gain access to the area.

Depending on your hardwire kit, you will need ignition switched 12v only, or a constant 12v too. In the fuse overview I've marked fuse spots that you can use. If you're unsure if this is correct for your car, you can measure with a multimeter with the iginition switched on and off. Switched 12v is yellow, constant 12v is red.

My fuse kit came with 5A fuses, and it has a spot for the old fuse to go into. The fuse taps won't work without both fuses inserted.

Connect the fuse taps in the correct place in the fusebox.
Cut away some of the slack in the cables from the hardwire kit, so your ziptie job won't look too messy.
Expose enough wire with a cable cutter or knife.
Use your crimping tool to crush the splice from the fuse tap against the exposed wire. (a metal barrel is clamping down on the wire in the splice, you can look and pull to check if it's attached properly)

It might be a good idea to have some spare fuses in the car if you managed to screw up the wiring, so you don't get stuck without a working speedometer or radio.

Step 8: Connect the Ground

The ground I decided to use is right by the fuse box, to the left.

This was a bit of a pain in the ass. I had to lie down on my back and use a slighly long screwdriver. By lifting your body via wheel (take the key out so it can lock in place), you will be able to see the screw.

Unscrew the Philips PH2 screw, and attach the fork or ring terminal of the ground wire to the screw.

Retighten.

Step 9: Check If the Dashcam Is Working

My camera didn't start up immediately, I had to turn the key to get it going.

If it isn't working, it's time to get that multimeter out, and check if there's any power at the fuses.

Step 10: Reinstall the Trim

Installing the trim is the reverse of removing it.

When you were removing the trim, you might have deformed the metal clips a bit. Slide out the ones that are loose, and pinch them hard enough to bend the metal a bit tighter, and put them back in.

I routed the cable near the front of the trim, because that's where it seemed like there was space for it. It might be a good idea to use clips (usually included with the dashcam) to attach the wire to the metal frame, or apply some padded tap to reduce the chance for rattling from the wire being loose.