How cabn I test some ICE without connecting it up in the car
 

As title?

Connect it all up out of the car and then connect to a 12V battery via a fuse, either take the battery out of the car or go and buy one / get one from a scrp yard

Via cigarette lighter connection, no need for another battery that way :cool:

A PC's power supply can give out the correct 12v line that will feed a head unit.
You will need to cut one of the molex connectors of the end to wire the correct wires though and its recommended you know which lines give 5v and which give 12v.

You should be able to find info on the net if not.

From here just connect the speakers et voila. Amps should be the same in theory.
If you struggle try www.talkaudio.co.uk as some people run HU's off PC power supplies in their home (dont ask why !!)

Or just use a benchtop 12V DC pwer supply (most should handle the current), obviously it helps at this point if you work in electronics!!

You can use a 12v car battery charger to power it up. Well I have done so in the past anyway. They can only supply a few amps so a head unit should be okay but don't try it for high power amplifiers and the like.