Changed car amp and now have wiring speaker noise? Help!

Hi I have had nothing but problems since my concert failed on me ! I changed it for a single din DVD stereo . Finally got that to work ( only switches off manually not with ignition , as not sure how to tap into fuse box) then the problem with the rear speakers not amplified . For love or money could
Not find original amp so wired four new cables for each speaker to HU . Didn't sound great at all so I have a old amp and wired that into the glove box(the lid then broke off the glove box!) it was ok except it only had input for one set of speakers not two. Also only one RCA input. (need two one front one rear) so brought a shark 1600w four channel amp from
A mate. Wired it up as the cables were all there . So all fine sounded great till I put the key in the ignition . The wirling noise started then when the engine runs ( once I had to install a new battery other or died, hoping it wasnt the stereo. Dot think it is as the amp powers off when you run the stereo off so should only be on standby and drawing very little power like a internal
Clock or security light) you rev the engine and it comes through the speakers ! I tried a different ground (several) no luck tried moving the RCA away from ever interference I could think of still nothing! It is the amp as when plug the other one back in its fine no noise at all? I've tried one speaker on its own basically everything I can think of!
An y help without paying anything would be great!!!
Thanks
Martin

That maybe the problem! It is a brand new amp, and i thought it would have worked! i mean it works but with the interfernce. As soon as you turn the key on to on (not starting the engine) the pitch noise can be heard from the ampped speakers.

Would this help?
http://www.amazon.co.uk/Av-Link-CAR-NF01-Skytronic-Isolator/dp/B000NVWB9O/ref=pd_cp_kh_0

I've been told it would and that would be cheaper than shelling out on another amp!
Just want it to sound ok not blow my ears off!
Maritn