You are not alone there, its a common issue. I have an alpine unit and it was a pain to fit. I'm worried cos I know i'm gonna have to take it out next weekend, which means putting it back in again. Anway, here is something I copied from a post I made a day or two ago. It may help you slightly, but the main thing is patience.
There is quite a bit of space down to the left and right behind the heater controls. You need to push as many of the cables down there as neatly as possible. Only leave the ones that need to actually go into the back of the HU sticking out. Then you carefully push it in trying to push the spare cables to the side. Its tricky but if you have the time/patience you will get there...eventually.