Nuts and bolts that are exposed to the elements often get rusted or corroded which causes them to seize. Bolts on your car have the added effects of heat and vibration that further add to the problem. If you need to remove a seized bolt, there are several tricks you can try that may loosen it up.
Apply penetrating lubricant onto the nut and bolt that are seized up. You will need to leave it on for a minimum of several hours; overnight is ideal. Use a rubber mallet to tap the bolt and help work the lubricant into the threads.
Use a tight-fitting wrench and try to leverage the bolt. Increase your leverage by extending the handle of the wrench with a piece of piping. You can also try an impact wrench to force the bolt loose.
Apply heat if the lubricant and leverage aren't working. Wipe off any extra lubricant as it can be flammable. Use a torch to heat the bolt directly, but be careful not to burn the underside of your car or get too close to the gas tank.
Cut the bolt as a last resort. There are several hand-held tools that will accommodate the tight space. Try cutting through the nut first then hammer the bolt out.