I am sorry to learn about this situation.
Start with first things first: If you have a court hearing on Monday - make sure you go. If you don't appear they will get a "default judgment" against you for the full amount they are asking for. You can contest this later based on improper service, but you are going to start with a position of having a judgment entered against you!
If you want more time, ask the judge for more time to prepare a defense when you appear on Monday. With bad service, the judge should give you additional time to prepare a defense (be prepared to go forward on Monday just in case they don't but most judges will give you additional time if the service is faulty).
For help in how to defend a small claims action - use the CA Courts self help site (it is actually very good): http://www.courts.ca.gov/selfhelp-smallclaims.htm
As far as the claim itself - did your credit card payment go through? (You don't say in your post whether or not there was a problem with your credit card, if your payment didn't go through, you need to make arrangements to ensure that the dealership gets paid - you can dispute that they failed to give you notice (both before they filed suit and with improper service of process), but you are still going to have to make the necessary payment simply documenting that an attempt to run a credit card does not waive a merchant's right to payment).
Most of these claims do end up getting pushed to mediation (even if you get more time from the judge, be prepared to go to mediation with the dealer). In mediation, if you haven't paid (if there was a problem with your card), be prepared to come up with a reasonable payment plan to quickly get the dealer their money.
When trying to settle a debt, creditors generally prefer lump sums over payment plans. They are often willing to accept an amount less than the full debt (the trade off is that they get a quick payment and don't have to worry about ongoing collection costs or administration). If you do not have the ability to offer a lump sum for something the creditor will accept (some will accept a small portion, while others want close to the full amount), you can try a payment plan, these are less satisfactory to the creditor (especially if they have a lien on your property already), but if you are willing to offer something with a reasonable chance to get the creditor a large amount of their debt back, you are likely to get them to accept it.
Whenever working with a creditor, make sure that you keep your communications in writing (if you speak to someone by phone, promptly send a confirmation letter to summarize your conversation), as this will help to ensure that there is no confusion later on, and you will be able to enforce your settlement against any future collection efforts.