from what you have explained, there is a high possiblity that you already have a amp in your car
i am going to assume a couple of things here, so double check everything before ordering
using crutchfields 'outfit my car', i entered that you have the jbl audio system-- double check wheather you do or not.
according to that, you can use the factory amp, so you will need a adapter like this:
http://www.crutchfield.com/p_1207081...s.html?tp=2977
and these:
1 to go from computer stereo audio to rca output:
http://www.radioshack.com/product/in...ductId=3449316
2 of these to split up the signal to the front and rear speakers:
http://www.radioshack.com/product/in...ductId=3295840
and 1 pair of these to connect the y adapters and 1/8-rca adapter(this is a 2 pack, so you would just need this one item)
http://www.radioshack.com/product/in...ductId=2102665
if you shop around, you can probably find better priced items, but this should give you a good idea of what you need.
now, if you do not have the jbl audio system, you will need a amp-- (search for motorcycle amps on ebay--they're cheap and small and should work)