Bad service, no communication and ripping customers off

In March last year my bakkie wouldn't start so it was taken to BJ auto clinic to be checked out, we requested a quote as well as to keep the cost of repairs as low as possible as I don't earn a great salary. After a couple of weeks of enquiries as to the progress they notified my verbally that the computer box was faulty and had to be replaced at a cost of R6000, I then approached my employer who agreed to help me pay for the repair so I gave them the go ahead, several weeks later with no response as to their progress they informed me that there was a problem with the cylinder head and that it had to go for repairs too needless to say I gave them the go ahead as I needed my bakkie. After 2 and a half months they finally informed me that it was ready so I went to fetch it and instead of receiving they keys I was given an invoice for R17640.51 and promptly told the invoice must be paid in full before I can get my bakkie back. I have made several offers to pay the amount off which they rejected, I sought some legal advice and was informed to open a case at the police station but to no avail. I know he is currently using my bakkie for is business getting spares and removing *******, he has also threatened to strip my bakkie to cover costs as well as demanding for the vehicles papers so that he can sell it