MY99 Wagon.
Just upgraded my speakers and engine noise feedback is more apparent through speakers.
Anything obvious to look for?

if u have put a amp in u need to run the power cables down one side of the car and the rca cable down the over side if they are t oclose u get bad feed baxck hope that helps

big dan

Ground everything to the same point and upgrade the headunit earth/negative.

Make sure the 'gain' control on the amp is not maxed out (as low as possible while still providing decent volume). The cable and earthing advise by Dan and P1 is seconded.