You will need a LOC unless your amp is capable of taking speaker level input vs RCA. The LOC will give you control for getting it dialed in as well versus just having raw signal (good or bad).
My installer had me purchase the T Harness from LLJ (still waiting to receive it at this point) as nothing else was available to him currently. Metra, PAC, etc either didn't have anything he could find or there was nothing available to order.
He could cut and solder but the harness will make it neater, faster, and easier.
Rockford 5 channel amp I use to power focal rear doors, focal front speakers (separate with crossover) and kicker sub:
https://rockfordfosgate.com/products/details/t1000x5ad/
Good luck and enjoy your much better sound when you are done.