interference in car speakers

Hi, I drive a 1995 Honda Accord, I have recently replaced the stock speakers, and installed a 4-channel amplifier to power them. It was working fairly well, until today I put in a sub box with another amplifier to power the subs. Now when the stereo is on there is interference coming through the speakers, partly static, partly a high pitched squeal. When I accelerate the squeal gets higher pitched and louder. Now I've read around on the internet to try and figure out the problem, I just want to pin point it before I fork out money to try and fix it. I have read about ground loops which could be the problem however I don't really understand what they are, also I've considered that it could be a problem with the alternator not being powerful enough or something. I've also read that I could try a noise-canceller, or something like that although I don't know anything about them. I'm just posting on here in the hopes of finding out exactly what the problem could be. Any help would be appreciated :)

Just curious...What type of radio...? Common with pioneers...! That link above should work great...You can do it temporarily to see if it cures the noise by just wrapping a wire around the rca at the stereo and touching the other end to the radio's chassis...!...If noise disappears...Do it properly as in Mr. Idiot post/link...!M.E.C.P & First-Class

Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!

It's a pioneer head unit lol. I'm getting a new Sony one in a few weeks but I didn't want to have to put up with the noise til then. I checked out that link and decided to just take a chance on it so I opened up the head unit and poked around with a meter and sure enough there was a blown fuse. Shorted it out with a strand of wire and now it works beautifully. thanks for the help!