Hello and thank you for your question.
It is unfortunately not surprising the police will not intervene. The reality is that because your dispute ha arisen in the context of a contractual arrangement between you and the dealer, the police will generally presume the dispute to be essentially a contractual business arrangement gone wrong and therefore a matter for the parties to sort out either between themselves or by civil legal proceedings.
What you need to do is consider bringing a claim in the magistrates court. Based on the law you cited it appears you are based in South Australia and that being the case the following link provides information about bringing a claim before the magistrates court.
Assuming your claim is for less than $12,000 then you can bring your case as a minor claim which fortunately should mean it follows a less formal process which is intended to allow parties to bring proceedings without the need to engage a lawyer.
Based on what you have said it does appear you have a strong prima face claim and it would appear the dealer will have a heard time explaining why they haven't either returned the vehicle or paid you whatever your arranged share was, and this being the case it would seem likely to me that they may well back down and simply pay if you instigate proceedings, or at least attempt to settle with you.
The main danger you may face is if they don't have the money any more, in which case you may ultimately struggle to recover the money since you can only recover money that a party actually has, but if you obtain a court order you can take further action to seize money in their bank accounts, or have the sheriff attend their property to seize and ultimately sell their assets to meet your judgment debt.
I trust the above assists.
Good luck and please rate my answer.