Are you on a budget or no?
If not, I say go with a good kit, makes life a lot easier.
If so, then I would say that you will need a good harness that hasn't been hacked into that badly to start with and also check the wires to see if they are corroded. If everything checks out, then you can go from there with manuals and such for wiring diagrams.
No budget, then check these out
Wiring starts on this page and goes for a few others