You have to replace a V-Belt, and you can't get to it.
Don't Panic, read on.
Step 1: How to Sneak in a V-Belt
V-Belts are commonly used to drive the accessories on the engine of a car. The common configuration shown has the crankshaft pulley driving the water pump (vital for cooling) and alternator (charges the battery) with V-belt A and the power steering pump and AC compressor driven by the crankshaft pulley and V-Belt B. The crankshaft turns, and everything turns, you have heat, AC, power steering, and you don't run down the battery. Most important, you don't overheat the engine. As things wear out, you may have occasion to replace one or the other. It's best to replace a belt when worn instead of broken, that way you're not in a pickle.
I've seen this configuration on cars by Geo, Volvo, Mercedes that have the engine in a fore-aft direction. It shows the belts and pulleys as you might see them looking through the radiator towards the engine and everything behind it.
Step 2: You Can't Get There From Here
So you have a bad belt and look, you can't get to it because the pulley that you would adjust to slacken belt B (the light one) won't play nice. If you lack the leverage, heat, lubricant and time to move it. A (the dark belt) is too small to loop around all of the pulleys ahead of it, and the motor, mounts and such are behind belt A. It looks like you can't get there from here. But you can, read on.
You can slacken the alternator tensioner to get belt A off of the alternator and the water pump, or just cut it if it isn't gone already.
Step 3: The Sneak, Part 1
With the old belt A out of the way, place the new one (you got it right?) as shown on belt B. You will use the starter or some muscle to move one side of belt A around the pulley, but not the other one. How to make the crankshaft pulley turn just a bit (say 90 degrees or so)? You can apply a wrench to the crankshaft pulley, any pulley still driven by a belt, or by using the starter to crank the engine for just a fraction of a second. Whatever you do, keep fingers out of the way.
By observation, you can tell which way the engine turns when cranked. Don't turn it the other way, bad things can happen.
Step 4: The Sneak, in Progress
This shows the sneak in progress, with the belt partially rolled into place. You're almost there, carry on. Be sure not to get both sides of A between B and the pulley. It looks hard on the belts, but they stretch. I've done this a few times, never damaged the belts.
Keep turning, now you've snuk!. You should be able to put the new belt A around the alternator and water pump, tension it and drive away. It wouldn't hurt to inspect the belts immediately and check the tension a little ways down the road. Hope this can be of help to somebody.