ABC V6 is the 2.6L 12V engine, correct?
If so, the thermostat is located at the front face of the engine, below the LHS camshaft timing gear, under an aluminum housing. Pretty sure you have to remove the toothed belt and loosen the non-toothed belt to get that cover off, but perhaps you can get away without actually removing the timing belt...
/daytripper '00 s4 6spd