Well most radios that are a "single din" unit are a universal fit for the most part. Unless the previous owner failed to use the proper installation brackets (came off the factory radio) things won't work too well. If you take a look at the back of the radio, to pull it out pull off the three HVAC knobs and unscrew the two screws under the knobs. Then proceed to pull the unit off (grab the underside of the panel the best you can by sticking your finger into the gaps behind the HVAC control knobs and firmly pull outwards.
Depending on the mounting method used by the previous owner, the radio
should be held in by 4 screws (two on each side of the radio). Upon removing the screws slowly pull the unit out of the dashboard and check the back.
The default aftermarket headunit installation kit for a ralliart with the sun&sound package will have a little adapter that plugs into the factory harness that looks likes like
this. If it doesn't have this adapter I highly suggest buying one as it won't function properly.
Upon removing the headunit there will be two screws (sometimes one) holding the 2 mounting plates (assuming the unit was properly mounted) to the radio on either side, remove those and replace the head unit with the new one. Remember you will have to either remove the old wiring harness adapter from the previous head unit and strip/connect the proper wires to the new unit to adapt the new one to fit the factory mitsu radio harness. When this is all done reverse the un-install steps and have fun with the new head unit.
PM me if you have further questions but there's plenty of info in the following thread:
Link
Searching is your friend.
-Jason
