First explain the situation and try to work something out, but if they won't budge try this. It is not a guarantee of success but there are cases where it has worked before and you have to be brave and not take the first thing they offer.
Find out what the car is worth at the moment (It is bound to be a lot less than you still owe). Then phone the company up and tell them you can't afford it so either they can reduce your total amount repayable or they can come and take it away. Then and this is importatant. Tell them you await a call from someone at their company authourised to make such decisions and you await their call. Then politely say goodbye and hang up making sure thay have your nunber.
Chances are they will phone you back with a better offer. Refuse the first offer, they can go lower than that. Tell them it is still out of your reach and they can reposses the car. You want to get then down to somewhere near it's current true value.
Why does this work? Well it will cost them money to collect your car and remarket it at an auction or sell onto a dealer. The LAST thing they want is the car back. This is a good lever for negotiation.
For everytime you tell them you can't afford it, tell them you await their call from an authorised boss or supervisor. DO NOT KEEP CALLING THEM.
You may have to pay a little more than it's current worth but I read of one man having £14000 of repayments slashed to £8000 using this technique. Tell them you can't afford it first and offer for them to take the car back at the end of the first phone call. Do not offer that straight away. Try renegotiating a better deal first and use repossession as your trump card.
Good luck.