grass - Creating a raster of the residuals of a regression between two rasters

I am trying to create a raster of the residuals of a regression between two rasters. i.e. I would like to carry out a regression of one raster against another of the same extent and plot the residual for each cell of the raster at the same resolution and extent as the original rasters.

It seems the GRASS 7.0 has a function called r.regression.multi which would let me carry this out but I have zero experience in GRASS and I can't even get GRASS 7.0 to work (GRASS 6.4 works fine but doesn't have r.regression.multi so is useless!)

I currently have the files in almost every format possible (Idrisi raster, TIFF, ASCII .grd in R) any help would be much appreciated!