On a trailer equipped with ABS, where is the malfunction lamp typically located?

49 CFR 393.55 (ABS malfunction indicator requirements); AAMVA CDL Manual Sec. 2.18.3, p.2-35

Because tractors and trailers are often mismatched units coupled together, each has its own independent ABS malfunction indicator. A driver who only checks the tractor's dashboard light could miss a trailer ABS problem, so knowing to look at the trailer's left-side lamp during a pre-trip inspection is a practical safety step.