During Toff roll say 10 to 5 Kts. prior to V1, the Reverser Unlocked light of one engine pops up
If the thrust lever retards automatically then you are in reverse on that engine and you reject fast. If there is no movement of the thrust lever then you are in forward thrust and you continue. Sort the problem light when you get time. Any abort that close to a real V1 on a limiting runway length - especially if a wet runway, is asking for trouble. Sometimes you have to make a critical decision and wear it.