Written agreed refund not released

On the 17th of June I had a connector plug fitted to my vehicle before traveling the same afternoon from JHB to CPT. At Beaufort Wes, due to a malfunction of the plug, I was forced to halt my journey and search for an auto electrician. The mal function visibly and clearly due to poor workmanship and incorrect installation. I brought the matter under the attention of the owner and management of Station Auto Sound (Ontdekkersweg). Agreement was reached in writing that I will continue to pay for the part but that will be refunded for the labor (R300). Despite numerous unanswered phone calls and emails the matter remains unresolved.