You could alternately run a seperate earth along with the power cable. How about running a temporary earth from the battery to work out if the amplifier is damaged or inop.
As to earthing in the boot. If you find a bolt within the surrounding area that goes into the chassis of the car, remove the bolt if you can, remove the paint under the bolt with some sandpaper taking care not to remove too much..thats not required, treat with rust killer, and loop the cable too it. Fasten bolt up, and con check it with a multimeter to find out the resistance to the earth. You then know that its hunky dory..and connect up to the amp.
Job done..