I would have to see both your mediated agreement and your settlement agreement to fully advise you on this. If the mediated agreement truly did not offer a date or any other guidance regarding when you were to begin paying half of the car note, it is certainly arguable that you are not in contempt.
I gather you made the big mistake of not having a lawyer, as a lawyer would have insisted on dates in any agreement. Absent a date, a judge would have to look at the agreement, guess what you and he meant, and decide on contempt. Since no one here has seen the agreement, predicting what a judge might do is impossible. Likely, if the agreement said pay half the car note, a judge will find that should have been done on the due date, so you may be in contempt. At this point, pay a lawyer to review your papers and advise you.
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Without dates enforcement will be difficult, although if this went back to court the judge would probably create one. This seems like a situation in which you both would be well-advised to work it out through direct negotiation or some form of alternative dispute resolution (like mediation).
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