Stevie Wonder is a fixture of famous-synesthete lists, usually asserted flatly. Chasing the sourcing turns up little: passing mentions in secondary articles, an often-cited but hard-to-pin claim about experiencing color through sound, and no substantial firsthand account in his own interviews that we can point to with confidence.
The claim is not implausible (chromesthesia does not require sight, and blind synesthetes experiencing color are documented in the research literature, which makes his case conceptually interesting), but plausibility is not evidence.
We file him as disputed pending a primary source. If one exists, we will upgrade the entry and say exactly where it comes from; that standard is the point of this database.