When doing an LS swap into a chassis of a vehicle that never had an option of an LS engine, you have a few options for wiring that you can go with. Sometimes that engine that you grabbed from the junkyard will have a harness that it came installed by GM with, and if it doesn't, you can hop on eBay or head to your Facebook Marketplace and find one with ease. Most are relatively inexpensive depending on the year of engine you are needing to control - usually ranging from $150 - $350, this is the most cost-efficient way to bring that swap to life, but is it the best?
There are tons of companies out who have realized the desire behind LS swapping. It's one of the easiest, most cost-efficient swaps that you can perform, and because of this, it has tons of how-to documentation for making it work across countless platforms and models. During my time in wiring, I have seen many companies sell clients "plug-and-play" harnesses, but what that really means actually has two different meanings. Are you wanting to plug-and-play the engine and transmission to get it to start and operate, or are you wanting to plug-and-play the engine and transmission AND have it communicate with the car properly, providing you the essential operation information that an OEM-installed powertrain would have?
Many of the "plug-and-play" harnesses sold by companies are actually of the STANDALONE variation. This naming is based on the idea that the components swapped are standalone, or separate, from the vehicle as a whole. Standalone harnesses typically do have some flying lead wires that can be connected to the swap vehicle for basic integration of things like gauge information, brake switch position, and check engine light, but if you remove your factory wiring and install a standalone harness, you will lose some of the OEM functionality of the vehicle, and functions lost increase as the production year increases. This is because vehicles have moved from more mechanical operation to electronic operation. In the past, vehicles had mechanically driven gauge needles which were replaced by gauges driven by variable voltage serial data. Since that change, gauges have become driven by a table of addresses and constantly changing values in a technology called CANBUS.
Now that we have touched on both factory/OEM style harnesses and standalone harnesses, INTEGRATION harnesses are a style of harness that connects to your factory vehicle wiring as a true "plug-and-play" unit that does not require much, if any, end-user splicing or connections to be made other than factory connections that were disconnected when removing the harness for integration. This makes installation seamless and trouble-free while providing the best swap experience and maintaining the most functionality by combining the original vehicle with its new powertrain. This swap style will either use the factory vehicle wiring harness or a new production replica that will have the new LS engine and transmission wiring combined, creating a single harness.
It should be noted that there are companies and individuals who will take an OEM-style harness and convert it to a standalone-style harness for you. This will either require you to provide or purchase the OEM harness, as well as pay for the conversion. There are many people who will convert harnesses quickly and cheaply but will not provide a good product, so care and judgment should be used to ensure you are receiving a product that is of good, safe quality.
So which is right for you? Used factory OEM style harness, standalone, or integration?
If you are trying to save the most money and do not mind unneeded connectors, poor routing of wiring, and needing to add some wiring, an OEM-style harness is a good solution. If you prefer a new harness or OEM with better routing options and unneeded connectors removed, a standalone harness is what you need.
If you want the best option that will require nothing aside from installation while providing an OEM-style appearance and function, the integration harness is what you will want to use.