Congrats on making an event, it is a lot of fun. My only first hand experience with the Sonic is with the hatchback and not on stock tires, but most of this is universal advice.
The first thing to try is to adjust the air pressure in the tires (pretty much the only thing you can change stock). What pressures did you run? I've found that the car will oversteer, as you describe, with too much pressure in the rear. You can drop the air pressure in the rear and find significantly more rear grip and avoid the over rotation. Anecdotally I have heard it is more difficult to get rid of this completely with the stock tires. That leads to the only modification I might recommend if you plan to autocross a lot. Get a good set of grippy tires - I'll need more info about your car/plans to give any specific advice here.
Now know that the oversteer you experienced is a good thing! Actually the ability to get the rear end to rotate is an important aspect of the Sonic's handling balance that will really help on course. Most front wheel drive cars, or really most cars in general, are tuned to avoid oversteer for safety reasons. So most drivers of these cars are trying to get rid of push and help the car rotate, this is not necessary with the Sonic as it handles great right out of the box.
Yes, I actually am planing on competitively next year.
Lowered and rear brace. Problem solved as far as the handling goes (on the cheap anyways)
Just start dumping performance parts on it. It can hold its own
Subsonic, I know you are trying to help...but get a bit more involved and you will find that throwing random parts at the car is bad advice. In fact the rear brace would make things much worse in this case, not better. The rear brace makes the car more prone to oversteer.
Being competitive requires a bit of knowledge about the rules and how modifications will place and affect your car as well. The Stock category ruleset is currently going through a major overhaul right now, so everything is up in the air until the rules are finalized for next year. Sub, you would fit into the ST* category, I wrote a letter to include the Sonic in that class...I've been emailed that the committee response will be in the next communication (around the 20th of this month). I'm hoping it makes it in
Anyway, I've probably autocrossed the Sonic more than anyone around and I've competing in general for many years. You can read my build thread about some of my adventures here:
http://www.sonicownersforum.com/forum/build-threads/7160-johns-autox-everyday-sonic.html I haven't been keeping it up to date lately.
