Hissing while car is on, is the alternator noise.. Get a noise filter installed.. Can get one from Jaycar.
Secondly, rear speakers bouncing ? Make sure all the screws are tight and securing the speakers to the base / panel that its on.
Make sure the panel itself isnt dodgy and running around.
You didnt mention anything about the amp, how is the amp mounted ?
The clips would be around at a wrecker or dealer, try ripping the trim open first, i found most my clips just fell inside lols.
Take your time, tune your headunit aswell.
Would help if you stated what gear you have man.
Let us know.
For some more advice or stuff that we cant answer, ask Chris Rogers.. audio guru.
MRXTCZ