There is supposed to be a speaker bracket back there but I don't think that there are any dealers that sell them. Your best bet would be ebay or somebody local that might be parting out their civic.

I happen to pick up a set from a dealer, but the thing I don't understand is that 6.5" speakers (I got kenwoods) do not fit. I mean the diameter of the bracket is larger than my 6.5" speaker so I could only screw down one hole. If I use a 5.25" speaker with a 6.5" adapter, it fits, but I have to screw down new holes. I thought it would be a simple job, but apparently these rear speaker brackets are not kenwood friendly.