How do i get to the spark plugs on the back side of the engine. It looks like I might get to them if i take the alternator off.

On all GM 3.1 L engines(including Buick Century), (be sure the engine is COLD, so that the steel spark plugs do not strip the aluminum threads of the engine block), the procedure is the same. Remove the 4 bolts holding the dog-bone shaped upper motor mounts that connect the engine to the front frame(the engine will not fall), above the radiator, on each side(two bolts for each mount). Then use one of two methods. 1. Using a tilt tool, available from Snap-on, or other parts houses, attach the tool in place of one of the mounts, and with the transmission in NEUTRAL(This is critical!), use a socket wrench to draw the engine forward to allow access to the rear spark plugs. The access will be a little tight, but all three rear spark plugs are now accessible. 2. Some technicians will use basic physics to attempt the process without the tool. Using that method, the same mounts are removed, but the vehicle is placed on level ground. With the transmission in Park(the opposite of the method using the tool, above), the emergency brake is released, and the vehicle is pushed forward, until the engine tilts forward(may a strong physique, or an 800lb gorilla, whichever is readily available in your immediate vicinity). Good idea to reset the emergency brake, to hold the engine in position, until the plugs have been replaced. You can then push the vehicle backwards until the motor mount holes match up again(after releasing the e-brake) and re-insert the motor mounts, and bolts. With a bit of experience, all three plugs can be replaced in less than 10 minutes. Good Luck!