i have an aftermarket head unit.... it is fairly easy.... but if you dont know what you are doing... just have a new one installed.... make sure you get a head unit filler from JLTroy so everything is flush, and a harness too...... click my pics to see a b15 with an aftermarket head unit
Ok it's been a LONG time since I've opened up my dash, but I'll try...
Open the top dash compartment and grab it by the hinges. Pull up and towards you. Don't force it though. The whole thing should come out.
Then see that little compartment below the climate controls? I'm not sure, but I think that's where the CD changer goes if you got it. But anyways, see the dash panel around that compartment? Find the bottom corner of it and pull on that too. That comes off.
Now everything is exposed. I think the headunit is held in by 4 screws. I could be wrong though. Unscrew those and you're good to go!
K. Hope I've helped. Just remember most of the interior panels just snap on and off. But be careful and don't force it or you might break something!