are you kidding bob? Drill holes for a bloody stero wire? Secure stereo wire to the undercarige? Can you say ghetto? Sorry Bob, don't mean to be rude but thats a really bad and hacky idea (espeically nasty thing to do to a nice mk2 like your 84). No need to be removing carpet or seats either. Running amp and rca wires to the rear of the car is simple as can be, just do it the way the engineering gods at toyota do it. I've owned 4 mk2s and every one I've done it this way and never had a problem. And it never takes me more then half an hour and makes no ereversable changes to the car.
Run the wire from the battery. Take it to a gromet on the firewall on the DS, theres usually a spare one there I belive and the odd other you can squeeze extra stuff through. Next run it up through the dash and over to the DS kick panel fuze box area. After everything is run, come back and zip tie the power cable in the dash and ensure that its being there will not interfere with the pedals and their movement at all. Next partially or completly remove the interior panel that goes over the interior fuze box. Run the wire down to the floor on the inside of this panel, be sure it its on the side of the fuze box, not over it. Next remove the plastic trim that runs the length of the bottom of the door well, its held in by only 4 screws. The carpet is clipped to the body under this piece, gently pull the clips off the body and pull the carpet back a little. You will expose a big plastic tube that runs the length of the door. With a little patience you can run the wire through this tube (its a little tight as half the wires running to the back of the car are run through here), but if your lazy just pull the tube away from the body and run the wire underneath. You don't even need to secure it to anything here. Now we're at the bottom of the rear seat. This looks hard but is easy. Pull the rear seat hand rests off (just pull up on them, they snap off). Now the rear seat interior panel is joined to another panel that runs up the door well with a little clip right at the bottom. Slide this clip off and pry these two panels apart just enough to push the wire through. Push it till you can see it through the hole where the hand rest was. Grab it and pull the wire through. Now you can put everything else back together if you want, but the last step is to push the wire over the rear wheel well from the hand rest hole. Getting the wire into the hatch from here is easy and there are many ways to run it. I usually run it underneath the bracing thats between the two shock towers as I usually put my amps around the fuel tank access panel next to the spare. Voila, done, no permant mods, completley invisible wires, and it should only have taken you half an hour. Do the same on the other side for your RCAs and any other wires you need to run back there. You want to keep your RCAs away from your big guage power wire. The other side is exactly the same as the drivers side.