Extract contents of zip to 'gpl' root directory, say 'yes' to merge folders. There should be no files overwritten unless you have installed other add-on mirrors before.
Note: If you installed the mirror fixes before they were included in the installer (ie before 30-05-11), you may wish to back these up. They are '/gpl/cars/cars69/c13/erml.mip' and '/gpl/cars/cars69/c29/zrm.mip'
Simply delete the relevant mip file from the relevant car folder (eg abri.mip from folder 'c00'). The mod will then revert to the original file which is stored in the '.dat' file.
Don't forget also that the mod team have provided a hi-res mirror option that will display what you see in the mirrors in finer detail. Simply drop the /gpl/cars/cars69/mirror.mip file in to the relevant car folder (eg c00 for BRABHAM BT26A ZA-ES-ROC)
Full credit of course goes to the authors, all I did was erase their excellent work.
Many thanks to the mod team: Charles Mark, Paul Skingley, Remy Roesz, Tommie van Ostade and Stuart Bartosiak