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 rubbish, 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