Installing a new radio isn't that hard to do. You may find it more annoying to cut the HVAC/radio plate to accommodate the new unit (unless you bought a DIN plate).
There are two fused wires that power the unit; a light green with white stripe wire (only live when ignition is on) and a purple wire (always live). Your installation instructions will tell you where these need to be wired to your new harness. I left the fuses in place, even though the new deck will be fused in and of itself. A bit of extra insurance, I suppose.
The speaker wires are thin little things, unless they've been replaced in the past. Because you really should remove the center console to do your radio install, you will be able to trace each speaker's wire to identify them. If they are still original (wires and speakers) you may want to consider replacing them......no new deck sounds good with 17W speakers!
I can't remember; does your car have the fender mount antenna? If so, you'll have a wire or two in the new harness that you won't use. If you have or are moving to a rear power antenna, there's a "sense" wire that tells the antenna to raise or lower based on if the tuner is in use or not.
The new harness comes with its own ground. You'll see where the ground spot is with the console beneath (it's on the passenger side). The speaker wires will now hook directly to their respective places on the wiring harness as the instructions will describe.
Of course, disconnect your battery before you begin. Oh, and if you have an original headlight switch, have you considered adding a relay to the wiring to save it from meltdown?