One of the best things you can do is LABEL each and every connector you unplug, ...as to where it goes to etc. As 'complicated' as the underdash wiring may seem, ...once you have it labelled, ...it's really not that bad. You might even consider a drawing up a picture of how the wiring physically goes in too.
Make sure you have a copy of the wiring diagrams for your year car as well, ...they are also a useful tool. There are color versions floating around that may be available for your year. I believe one of the members on this board did some of them...