beefy PC speakers and powering them

I have some beefy oldish JBL speakers that take an input of 12V 1A DC (thats what it is rated), i do have a multimeter to test stuff, but i was wondering if i connected it directly to the car battery would it work (would the speakers only draw what they need?) or would it blow the amplifier circuit o nthe inside of the speaker casing, or would it blow out the speakers?

ALso i have built an audio player, that plays mp3s from a computer, what would be the best way to power it in the car? (direct from battery, alternator, 2nd battery, beefy battery like truck battery?) and how would i get all the voltages needed? like the +12V, -12V, +5V, -5V, +3.3V, -3.3V, etc, basically, how do i run an ATX power supply from a car.

okay i have decided on an inverter, just not which one, i am looking at three diff ones currently

http://www1.jaycar.com.au/productView.asp?ID=MI5030&CATID=&keywords=inverter&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=xxxxxxxxxx&Keyword2=xxxxxxxxxx&pageNumber=&priceMin=&priceMax=&SUBCATID=

http://www1.jaycar.com.au/productView.asp?ID=MI5058&CATID=&keywords=inverter&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=xxxxxxxxxx&Keyword2=xxxxxxxxxx&pageNumber=&priceMin=&priceMax=&SUBCATID=

http://www1.jaycar.com.au/productView.asp?ID=MI5060&CATID=&keywords=inverter&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=xxxxxxxxxx&Keyword2=xxxxxxxxxx&pageNumber=&priceMin=&priceMax=&SUBCATID=

an inverter (12 VDC to 100 VAC) will be the easiest way. Pick the inverter that has same power output (or similiar) as the ATX power supply you want to run off of it