2 of the 3 small gauge wires that come from the igniter atop the coil, cross over the top of the engine then dive down between the intake manifold and right side of engine block broke where they turn down between the block and manifold. They are very old and brittle. I want to replace them and was seeking any advice on wires that would be more resistant to engine heat and how to join them to the ends of the old wires.
My plan is to solder in new wires and cover all 3 with an insulating wrap. Does anyone know if the engine heat will melt solder and if so a different way to splice the new wires into the old ones?

 

Toyota never solders wires; only crimps. Do you think you are "smarter" than the engineers who designed your vehicle? Okay, it might be cheaper to crimp than solder, but it is always true that solder forms a very weak mechanical joint. You need to attach the wires to each other (e.g., twisting -- or much better yet, crimping) so well that you cannot pull them apart even without any solder on them. THEN you can solder them. So if you're going to crimp them anyway, just stop there.

Almost all wire you can lay your hands on now-a-days has THWN-2 thermoplastic insulation (or it's equivalent), which is even more temperature resistant than the 28-yr old stuff in your truck. So don't worry about it; the old stuff lasted close to 28 years, wouldn't that be good enough for you?

 

Do you know how long it would take some little old lady like my Mother to solder all the joints on a wiring harness.

Smarter then Toyota yes on some things like having the slackers at Dana build frames.

What I have started to a few years back to splice wires is to use a uninsulated butt connector crimp then solder then heat shrink.

In the great Salt belt just crimped connections even heat shrinked still tend to fail after one season.