Then get out and walk to Carl. Start mission, then walk back to car. Get in and you can then use the power plant/Stone Cutters hidden tunnel to beat Smithers to Burns mansion.
If you don't park your car correctly then you can't use the power plant short cut because it is blocked off.
This cheat works because as the shutters come down to block off the power plant short cut it hits the top of your car and stops until the car is moved.
Warning! If you park your car too far inside the power plant then you'll have to cancel the mission to retrieve the car because it will be trapped inside the power plant.
square x3, triangle (to use this cheat you need to be in a car and press L3 (The horn)) now you can go and crash into cars and get lots and lots of money. Note: you need the car jump cheat for when the police are after you for when they get close.