You will find much discussion and information in this thread about the project.
The most useful posts for installing the TomTop unit however are:
Blue8T6 - (Posts #79 & #80) which have excellent photos and notes on the correct procedure to remove the door card etc, identifying the 86 wiring loom and the best place to mount the unit.
wjamin - (Post #83) which has the correct instructions (ignore instructions in Post #80) and a useful colour coded diagram on how to connect the TomTop unit to the 86 wiring loom.
GTS Blonde - (Post #114) which lists my husband's comments and tips following his successful installation in my GTS seven months ago.
From what I remember of my husband's efforts, he studied the relevant posts very carefully for a few days until he understood everything, made sure he had all the necessary tools etc and set aside a weekend in the garage so there was no pressure (he actually only needed one day). My set up is still working fine.