My appologies for the misunderstanding; here is the command and output to the screen (for a volunteer who generated a lot of -1's):
3dFWHMx -mask mask+orig -input residual_error.fit+orig -detrend
++ 3dFWHMx: AFNI version=AFNI_2011_12_21_1014 (Jun 11 2013) [32-bit]
++ Authored by: The Bob
++ Number of voxels in mask = 33969
*+ WARNING: removed 237 voxels from mask because they are constant in time
++ detrending start: 39 baseline funcs, 555 time points
++ detrending done
2.76899 2.39622 2.09311
