The BEST way to get a good fit is to have the dash top off.
This allows your ot move the wiring/loom properly out of the way. There is a fair bit of space to the left of the unit where the loom comes up.
As for making space behind it, you wont. The heater ducts dip down at the back so your very restricted on what you can get in there.
On my last car i managed to get a fair whack in there, just had to position the wiring to the left side and then push the head unit in slowly making sure wiring did not get trapped.
Be careful with it tho, some people on here have managed to short things out and end up blowing fuses.