Not too difficult a job if you are mechanically inclined

. You will have to run a power wire from the battery to the amp of course (wire size determined by amp draw/length of wire and it MUST be fused within a foot or so of the battery BEFORE it goes through any metal) as well as a ground wire from the amp to the chassis. The easiest connection to the factory headunit is by using an amp that has speaker level inputs (just attach the rear speaker wires directly to the amp high level input) however if your amp does not have this feature you will need a
high quality adapter to convert the high level signal to a low level signal (adapter connects to the rear speaker wires and an RCA connects to the amp). Unless the amp has "signal sensing" turn on you will need a 12v source to turn it on. Some factory headunits have a remote output for an external amp however if yours doesn't you will need to either run a wire to a
switched 12v source in your fuse box, the power antenna lead if your radio has one (ONLY if it supplies power with all sources as some only work when the radio is the source) or you can get a speaker level adapter with a built in remote output (will require 12v power and ground).
Hope this helps!