SH&**^&TT
didnt read it properly man..
I thought u were about to replace ur stock head unit with the aftermarket..

In this case,
It should be really easy!Esp, when u change it to alpine again.
When u buy a brand new head unit, you should receive 2 metal things [looks like ice cream stick]
take off the frame
Slide those 2 things in [ Left and Right]
Pull it out then way you go...

NO NEED TO TAKE OFF SURROUNDING PANELS [MY bad]

good luck