It is really easy as im no clever person and I managed in my 'X' 126.
I had 4" sony speakers in the front, set in little speaker pods from any good motorist shop or e-bay, These can be fixed to the front wheel arch inside with short self tapping screws.
I also made a MDF parcel shelf with kenwood 6X9s fitted.
The wires are easily fed under the carpets and back seat.
If its a BIS it will have a place for the head unit if not you can get another plastic holder that fixes under the dash.
I did have a sub in the back for a while but got bored taking it out to check the oil!!
