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When you go junkyard searching or wherever you should look for a 2001-2002 harness and computer from a drive by cable truck. Those are the best years for harness's and computer operating systems. You'll also want to find a cable throttle body too.
If he want's it to work with the factory stuff in the 99 S10, You're best bet is to use 99-02 engine wiring and ECU. The 01-02 stuff is preferred though.