I've read that the full LED 2021 TRD Pro headlight assemblies won't work in earlier 5th Gen 4R's because of issues with the wiring harness.
Alpharex headlights are "5th Gen plug and play," up to model year 2020. For model year 2021 Alpharex developed and sells a custom wiring harness that allows their headlights to be plug and play on the new TRD Pros.
Let's set aside the "why would you swap 2021 OEM LED's for aftermarket" conversation.
Instead, the question is, does this harness enable pre-2021 5Gen 4R's to plug and play 2021 OEM TRD Pro headlights?
21-up Toyota 4Runner to AlphaRex Projector Headlights Converters | AlphaRex