Speakers: Im guessing these are components if they have cross-overs with them, are they the same size as the speakers that came out? if they are then you wont need any modification of the doors. If they are bigger then you can buy adapters from your Seat dealer or an ICE supplier im sure.
Earthing point: If you take the plastic trim off the boot(the bit with all the lines on it, there is some metal pins which are used to clip that plastic piece in place, you can get a ring connector, cut a section out of the ring with snips and then put it around the pin and bend it to stay there. This is how i did mine and with the plastic trim back on its got nowhere to go, also very easy to access.
The grommet: I used the one slightly further down the bulk head, i had a black wire coming through there already but i dont know what its for, i just pulled the wire till the grommet popped out, took a file and make the hole bigger, wrapped both wires with a thick layer of insulating tape and pushed them back into place so that the tape is stopping the wires from rubbing on the metal.
The wires: The easiest way to run the wires is to put them in the channel that runs under your door sills, through the huge space behind the rear door cards(3 door) or under the rear door sills(5 door) and then right to the back of the boot and out from the side under the carpet.
As for the actual wiring of the things, remember to connect EVERYTHING else before connecting the power cable to the battery, also make sure the fuse is in before you connect up to battery. If you dont earth it or you have a wire lying around, you could short it out and blow your amp or even start a fire.