I can hardly believe this one isn't on here. My cousin gave it to me like 4 years ago. Well here:
Look on your map. You see Wrigley Field really far away, right? Well drive on up there and park your can around it. It doesn't matter where. I'm not sure, but SOMEWHERE around the whole thing, there is a little door with 'TICKETS' above it. Go up to it and push triangle. The gates to the field will open and you can get in. You'll have to go inside and WALK up some steps on the far left in the back of the field. Then you'll go right and there is this huge pathway you've got to follow. Around the corner is a yellow car with two black stripes on it's front. There you go. Secret car. It takes like 5 minutes to get to it, but it's worth it most everytime.