Audio Systems: Engine Noise, land rover discovery, aftermarket stereo

Question
Info: 1996 Land Rover Discovery
Had an aftermarket Pioneer installed at Circuit City, but there is awful engine noise. The engine noise only comes from the aftermarket stereo, not the factory deck. The reason I am even swapping out the decks is because the factory deck turns off and on after about 20 min of operating. I thought this may solve the problem. Now I have to deal with the whining engine noise of the Pioneer through the speakers. The tech tried a noise filter and alternate grounding, both with no luck. I have scoured the net looking for solutions, I've got things like running the radio directly from the battery, both ground and power wires. Before I delve into anything I thought maybe some outside advice would be in order. The CC techs said the problem is coming from the factory amp. Their solution is to either replace the amp with another factory amp ($300) or rewire the entire vehicle ($300). I am on somewhat of a budget and would like to solve the problem with the least amount of money. Either rewiring myself or fixing the in and out reception of the factory deck. Also how could it be the amp if the factory radio sounds fine vs. aftermarket?

Answer
every time i have to do radio installation and a high end vehicle with  amplifiers i tell the customer that they need a soundgate interface if not we don't touch the car this little piece have save us many times no engine noise and good audio

http://soundgate.com/index.php?request=product.details&categoryid=188&productid=...

good luck