Really easy. Trace the stock speaker's outline on a piece of 1/2" plywood and cut the appropriate size hole to mount the 6.5" woofer. You can even mark the locations for the stock mounting screws on the plywood baffle for easier mounting.
I used the stock wiring into the door (spliced in at the factory amp location behind the glovebox) and mounted the crossover in the door. You can also drill some new holes to run the wiring. I'd recommend against messing with the molex (sp?) factory wiring connectors because screwing them up can be a real pain in the a$$ to fix. Try a search using "factory wiring" for more details.
