This is a dumb question, i've tried searching, but I'm not exactly sure what I'm searching for.

Suppose you are interested in putting a modern, turbocharged, fuel injected, into an older vehicle. for example, a CA18DET/ RB20DET engine(ecu,uncut wiring) into an older car predating fuel injection, What kind of wiring would be required to get it running. the cheapest method preferrable. would I need to salvage a relay/fuse box assembly from some car in a junkyard. are all the sensors in these types of motors, controlled by the ecu.

Also, suppose the "uncut" wiring harness is garbage, is there somewhere I can purchase salvaged, or even new wiring solutions, something like painless wiring harnesses.

I see these swaps of newer engines into older vehicles, and the electrical aspect baffles me as I haven't worked on very many fuel injected vehicles before.

While not the cheapest usually, a standalone engine management system will be simplest to install. It will control all the engine's functions totally separate from any body harnesses, etc. Sometimes they will come with their own harness and have built-in relays and fuses (which can easily be added on those that do not). Cars do not really need that much wiring to function. Modern ones have so much wiring because we are spoiled with a bunch of crap that we really don't need.

cool thanks!

depends on the donor, and the builder, the L28ET swap is very popula on the carb'd S30's only take about 6 wires and I found taking the donor Fusible link box made for a cleaner quicker instal, I later upgraded to megasquirt, after fabbing up the injection harness it only requires about 4 wires be tied int the chassis and the megasquirt system will of course work on any engine

Well you just need to understand the wiring in general. The main thing about a harness not native to the car, is that it has nothing to plug into, and no power. Basically you will need a couple of relays, one for the ecu and one for the fuel pump (ecu 20A-25A and fuel 40A-60A depending on how much power draw you plan on running with dif pumps etc). Next you look for a wiring diagram of the harness and feed relayed power to everything that is supposed to be fed when the key is turned on (usually ecu power and ignitor/coil power). The fuel pump or pumps will have it's own relay and is switched by the fuel pump relay signal from the ecu. Ground the harness as it normally would be, and run a power cable and starter signal to the starter. Don't forget the power supply cable on the alternator too. Go through a checklist and make sure most if not all are connected and accounted for and give it a test start.

Depending on how old the car is, you will need to figure out the fueling system and make sure it is adequate to feed and return fuel.

The new new engines are twice as hard cause they have key readers etc, basically a lot of added gizmos and security that gets in the way. Still managable.