| i'm using a 4X50watt amp that is about 10inchs by 10 inches. i've got it as far to the passengers side as possible.
before you have them do anything, i would just unplug the preamp cables from the amp and turn the car on--make sure to at least elminate the amp as the source of the whine!! then work backwards like i did by checking the routing of the preamp cable and the ground points for the amp.
actually since you're using the factory head, where exactly is the Hi-lo converter? is it at the headunit and you're running rca pre-amp cables all the way to the back of the car? or did they run speaker wires from the headunit to the back of the car then use a hi-lo converter closer to the amp?
sorry it's kind of hard to debug since i'm not 100% how the shop did things. since you're using the factory head, you need an amp that supports high level speaker inputs or a hi-lo converter to change speaker level to pre-amp levels.
in my situation with an aftermarket head unit, i have rca pre-amps running from the front all the way to the back. you might not have that.  | 
