Sure, here goes. A 2nd class lever has the fulcrum/pivot at one end, the weight in the middle, and the (upward) force at the other end. I'm not good at pictures, so I'll try to paint a word picture, instead.

Using two stiff metal channels (5' fence posts), I placed one end (the fulcrum) of the posts (the lever) one foot ahead of center-line of the rear axle, with the scale supporting the other end, which was five feet away from the fulcrum. That made the math very easy: by multiplying the bathroom scale reading times 5, I had the weight of the rear end. Because most bathroom scales read up to 250 lb., I was pretty sure that I was in range. For heavier cars the same principal applies, just use a lever long enough to stay within range of the scale. For example, if you expect the car to weight 2,000 lb, you can use a 10' lever. Note that you might have to tare the scale to account for the weight of the lever, but in my case, that was negligible (less than one pound at the scale). Oh, yes, by "exactly", I mean within 1/16".

If you do use this technique, make sure that no part of the level touches the ground or it will throw off the readings. Due to bending, I had to raise the fulcrum three inches off the ground.