This site should help with the wiring schematic. I find the site to be pretty cool in general.
http://www.bcae1.com/spkrmlti.htm
You could get much better results using used home theater/stereo speakers from a thrift store or CL. Car speakers wouldn't work too well acoustically without an enclosure.
That said, the original plan of car speakers would work
With an enclosure, yes. Without an enclosure you'll need speakers recommended for free air use (like in a car door that isn't properly sealed up (e.g. stock) or the rear deck). There are a few Thiele-Small parameters you can look at to determine if something should be decent in free air (such as the Q factors) but generally you should Google the speaker and "free air" or something like that.
If no enclosure and they aren't designed for this type of usage, they will sound so bad as well as be so inefficient that you would probably overload the receiver just trying to supply the power needed for a "moderate" listening level.
I bought Polk Atrium 4 speakers to use. I got them for $80.
http://www.amazon.com/dp/B00378KMF0/?tag=atomicindus08-20
Yard sales are another good source.
Thanks I was just wondering. I was looking at ceiling mounted speakers and they were about 100 a speaker, whereas I could get some car speakers about 100/pair.