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You need to contact the Department of Higher Education and Training (DHET) if you want to write your trade test. You can find a trade test center on their website here: https://nadsc.dhet.gov.za/site/Trade%20Test%20Centres.aspx
Alternatively, you can contact them on their website here: https://www.dhet.gov.za/
We can't help you any further than the advice given above.