yes, i have already connected the radio to psu 250W with a 60W speaker front right
first thing you need to know is that you need to get away from the watt ratings of a psu if youre using it outside of its intended purpose. Computer PSUs output voltage and amperage. Typically all PSUs will output 3.3, 5 and 12 volts. You want to ignore the 3.3 and 5 volt wires. The 12 volt wires are usually coming from a singular transformer, just split into multiple wires so that the molex plugs they connect to can evenly distribute the power where it needs to go. You arent using it for that anymore, so youre best off just cutting the molex off and summing them on something metal and insulated as to avoid sparks etc, but still get the full amperage you can from the PSU. If you dont do this, youll run the risk of trying to pull too much current thru tiny ass wires and thereby heating them past the point where their shielding is designed to protect them at, it melting or burning, then FOOM, fire...
Anyways, the deck needs +12v and gnd(which can also be obtained from the PSU) so does the amp. The deck will have a "constant" and a "switched" for +12v, and unless you plan on leaving the psu on at all times (HORRIBLE IDEA) youre going to run into the need of resetting your bass/treble etc etc if theyre digitally controlled inside the deck or find some alternate source of a tiny amount of +12v and a ground to maintain the internal memory on the deck and use the PSU for its main power.
the red and whites youre talking about are rca's for audio out from the deck to the amp. and yes, you then connect the sub to the amp. anyways, are you sure you understand the risks of playing with 12 volts in a home situation? Its a great idea on finding a way to burn everything you own to the ground if youre not taking precautions. If youre just cannibalizing a dead desktop pc to run some old car audio stuff in your house instead of spending a little bit of cash on a plate amp or a used PA amp, you had better know what youre doing man. It doesnt sound like you do since youre trying to use a 250 watt psu on a deck AND an amp. I use a 450 watt psu for testing car stuff just to see if it powers up and if itll play music at all, difference is that my 450 watt psu has been properly converted and i know how many amperes it outputs vs the drap on the stuff im hooking up is