I have a few things to add. Using a computer PSU in this manner will kill it eventually. But if you are going to cut off all of the connectors, I doubt you would be using a PSU that you weren't afraid to lose
. All but a select few ATX PSUs have minimum load requirements on the 3.3v and 5v rails that need to be met for the PSU to operate correctly. Also, wiring up multiple PSUs as been described would cause feedback in the voltage regulation circuits, ultimately leading to improper voltage regulation and eventual failure. I'm only somewhat familiar with car stereo systems but I would also be worried about the transient loads (going from a low load to a high load in a short period of time) stressing the main transformer and other components on the primary side of the PSU. But as long as it's a PSU that you don't care much about, I see nothing wrong here and is a nice use for a spare PSU.