After searching fruitlessly through many forums for a definitive answer I've finally done it myself.
My vehicle: 13 plate Fiesta Sport TDCI Van
The vehicle only came with front speakers 2 x main & 2 x tweeters in the front doors. I stripped the rear trim out and found the harnesses in place for the rear speakers. I purchased a pair of 6.5" dual cone Pioneers with the required spacers and connectors to plug into the ford harnesses and fitted them. No sound! There was no fade option on the standard fit Ford head unit. Read a few more forums including in the USA.
Downloaded a program called Forscan & ordered a ELM327 (USB) OBD adaptor from Tunel rat electronics for £15ish. After a bit of fiddling and getting used to the Forscan software I found the correct modules to change the settings on and hey presto! Front and rear speakers working with fade option now available on the head unit.
