Which aftermarket swap harness should I buy for a 5.3 swap?

First match the harness to your engine and transmission: 24x (Gen III: LS1, LS6, LQ4, LQ9, Vortec trucks through 2006) uses a 1x cam sensor; 58x (Gen IV: LS2, LS3, LS7, LS4, L76, L92, 2007+ Vortec trucks) uses a 4x cam sensor. PCM, harness and sensors must match; a Gen III PCM on a 58x engine needs a conversion box. That decides between a "24x" (roughly 1999–2006 Gen III trucks) and "58x" (2007+ Gen IV) harness, and whether you need automatic-transmission circuits (4L60E/4L80E, 6L80) or not.

Two kinds of harness:

• Standalone by engine — A standalone harness keeps only the circuits the engine needs to run, removing body-specific donor wiring. PSI Conversion lists 17 standalone LS/Vortec harnesses from $599 to $769, organized by 24x/58x and transmission.
• Vehicle-specific plug-and-play — Wiring Specialties makes 108 LS swap harnesses for specific chassis ($32–$5,359); check your vehicle's swap page for the ones listed for it.

Don't cheap out: Very cheap (~$150) harnesses with thin wire and knockoff connectors cause intermittent misfires, no-starts or worse; quality standalones use proper-gauge wire (TXL), new Delphi connectors and are tested.

Compare both makers and every harness on the tuning page.