Hi Gaz,
Try turning the gain on the amp down to zero,volume up to 1/2 way then turn the gain up a little but well below distortion.Start the engine and listen.
Or ,try tracing back the RCA cables in case they cross over a power supply,the same with speaker cables.
Or,check you have a good earth,which should be direct to car body from the amp.
Or try fitting a suppressor block in line from Halfords even.
What car have you m8?I have a 20VT,Iced of course! and also in North Wales,Bangor!...Geraint.