What is a good method to troubleshoot a dead short in a wiring harness? Are there any devices that would fail and create an injector ground-out?

https://www.fixya.com/cars/t11782315-fuel_injector_fuse_blown_keeps_blowing
This response has several possibilities.

Check your efi fuse does it look melted like if so
check wiring harness for injectors,and the injectors Short to ground in harness or shorted injector. common problem with injectors are shorts to ground. No garentee this is your problem but a issue with alot of Toyotas in the 90s and early 2000's

I worked on one with the same issue. Ended up being the harness burned at the rear of the engine on the exhaust. Burned wires going to coil pack and shorted when I moved the wires to the coil packs.

The injector ground circuit has to be shorted to ground, only two things cause this, the wire to the injector from the ECM is damaged and grounded or the ECM injector driver circuit is shorted out.

Not usually. They use the same power source but are grounded at different times by the engine computer.
Its possible one bad injector could damage the others, but that rarely happens.
An injector is an electric valve with a small coil and magnet inside.

It could be a short in the wiring-a bare wire touching metal, but sometimes an injector will go bad and short out blowing the fuse. Try unplugging all the injectors, then put new fuse in and turn key on. If fuse blows again, it must be a bad wire in the injector circuit. Each injector has a power wire and a ground wire, the power wire probably derives from the engine computer, and the ground wire definitely goes to the computer.
If the fuse didn't blow right away, try connecting one injector at a time (key on). If connecting one injector causes the fuse to blow-that is a shorted injector. It will have to be replaced.
If connecting all injectors did not blow the fuse, try the wiggle test on the wires, with key on. You may cause the fuse to blow by wiggling or moving a section of wire, and that is the likely source of the short. Separate the wires and examine them closely. Look for exposed wire, melted insulation, things like that. Look for where the wiring harnesses may chaff or rub against something, may be stretched or stressed. If you still can't find it, you may need to run continuity or resistance tests on each individual injector wire. For that you would need a multimeter, a good digital volt ohm meter (DVOM) at a reasonable price is a necessity for automotive work.

you mean the injector # 3 was dry ? Is that injector connector plug-in, wires ok ? do anything different ? Is harness ok ? Is plug wire ok ? you need to make sure you have power thru and to #3 , even new wires can be bad ! check for a short, continuity, and good ground. any codes stored in the ECM ?

I looked the Code up and it did not make sense because your engine does not have more than 6 cylinders. But it is an Injector grounding code. You can either borrow an injector wiring tester at a big parts store like Autozone or use a test light on each harness wire attached to the injector.

Do not ground the test wires to the block but use both wires in the harness because it is the harness that you want to test. If all the wires are dead, you will need to trace the ground wire.

Autozone.com has a free wiring diagram and fusebox layout for you by registering your vehicle at their site. It should help you find where the wires go to. It is possibly controlled by a Relay. Most people overlook the Relays because they think all the wires are Pos+ but there are a few Neg- wires that interact with the Relays and make them work.

You can just as easily have a Relay fail to connect either a power wire or a ground wire. So a bad Relay can break a circuit. There are many parts in the system, since you used a Scanner to get the Code 0212 you have localized the problem to the grounding circuit.

Check the connectors to the injector harness and you may have to do a continuity test to see if any wires are broken.

I am going to assume that you verified this by unplugging the injector and seeing constant power across the pins in that connector, and capmparing to the another good injector. if you have not done so. that is your first reading to take with a meter.
a breif description of how it typically works:
nomally, there is a single common for all the injectors. typically, it is a positive signal energised by the ecm/ecu either directly, or via relay. and usually fused. sometimes, on v8, and some v6's there are 2 commons. then, each injector is individually actuated through the ecu.
next test- if you find the common positive voltage, then the other wire should read nothing with the engine not runing. do an ohm check to ground. you will likely find that n4 only is reading continuity on the non-common wire. trace the harness to the main harness plug. it may be a disconnect in the engine bay, at the firewall, or a complete run to the ecm. unplug it. it may require wd-40 and some massaging fennesse. force is not the first answer to pulling it apart, and the clip may not release normally, reqiring a pick in there to unlatch it. if you still have continuity, then you have a short in the harness grounding between the 2 points. if you do not, then trace the wire from the connection point you disconnected, at least 5 inches away. you can do a pinout check, to find the correct wire. test all to verify you have the correct wire and then you will cut this wire, if you know you can reliably re-attach the wires together. before you do this, if it is not at the ecm test the corresponding pin at the other connection you unplugged from, to make sure you are not reading ground continuity (you may need to find the ground pin in that plug, as it may be the same plug that feeds the ecm ground.) then, unplug the ecm, and re-connect the disconnect, if applicable, and re-check continuity. now, you move to cutting the wire, if the ground is only present with the ecm connected. plug the injector back in and turn the key on. if you receive close to common voltage on the injector side of the cut wire and ground on the ecm side, then there is an internal short in the ecm/ecu. if the system uses a common ground, then all of this is inverted, with the positive battery/ign. as reference, not ground

check wiring harness for injectors,and the injectors Short to ground in harness or shorted injector. common problem with injectors are shorts to ground.