I just removed the stock engine from my 1999 Miata. My question is what do I need to keep? I want lights and gauges to work with my Ls3 swap

 

Luckily the engine and body harnesses are separate, the engine harness comes through the center hole in the firewall, and the light harness come through the outer edges. If you aren't trying to strip every connector and wire you aren't using, you can leave the light harnesses alone. If you want to keep the engine harness complete to sell, I believe the only wire that you'd have to put back in plate is the coolant temp sensor wire.

http://www.yorbalindamiata.com/image...g/2000wire.pdf

Check out pate Z-28 and Z-30. The big blue connector (I think it's blue?) that separates the engine harness from the body harness is X-16 in the wiring diagram, and if you look at the instrument cluster pages you can see that the water temp sensor is the only wire that runs through this. The oil pressure switch (pin 3e on page Z-30) must run through the transmission tunnel, since it goes through connector X-10, same as the speed sensor wires. If I remember right, there's no wires on X-16 other than the water temp sensor that you'll need to keep. I'm doing a custom cluster, so I removed the whole engine harness and sold it with the engine.

Beyond that, I just removed the whole body harness and started identifying connectors I don't need anymore. I'm deleting a lot of stuff, all audio, power door locks, air bags, rear defrost, side markers, all emissions, fuel tank pressure control, A/C, heat, blower motor, fog lights, etc, basically anything not necessary. Unwrap the harness, but leave it together with loose zip ties, or the white tape that is left under the electrical tape.
When I identify a connector I don't need, I follow all the wires through the harness, and cut it wherever they either reach another connector or a splice, and cut it near the source. The just work your way around the harness until you've gotten to all the connectors you don't need. Then go to the big connectors (some wires just pass through this main harness and don't have a component connector) and identify which wires on the large connectors you don't need and do the same thing. Just be careful with cutting wires, if you are cutting at a splice or if you aren't depinning the connector, then the wires needs covered well, especially if it's a 12V wire!

I've been making pinouts of the large connectors so it's easy to see what wires are needed and not. I'll organize them and share them if you'd like.

 

Thank you, this will be extremely helpful once I study it a bit longer. A pin out would be very helpful

 

Here's the quick and dirty pinouts, hope it helps!

 

These diagrams will be very helpful when I get to that point in my build. Thanks.