had a bit of a same issue when i attached my head unit. the space on the back is indeed limited. What i did in the first place was to squeeze the cables and iso connectors to the left side on the back, there is some small space there where all the cables would fit.
After a couple of weeks, i bought a parrot car kit (which has loads of cables and plugs), which i ended up squeezing under the black empty compartment, right behind the climate control unit. The cables that had to go up to the head unit found their way from the left back (again, same space).
Good luck!