You don't need to drill anything. Here is a cut & paste from something I wrote a while back:
The firewall hole is a bit tricky to find. If you open the hood, and reach back between the brake booster and the strut tower, you will feel a 1" dia. rubber grommet about 2-3" below the cowl. Just push it through the hole into the inside of the car. Then stick a long screwdriver into the hole and leave it there. This will help you find the hole on the inside of the firewall, when you are on your back on the driver's side floor.
For the ground, flip your back seat down. There is a large (14mm?) bolt on each side of the frame, near the bottom. Remove one bolt, and grind the paint off the surface around the hole. Make sure you are down to bare metal. Use a ring terminal to connect your ground wire there.
The remote wire I used is in the trunk. I tapped the red/pink stripe wire (power antenna) on the main RCU harness. To get to it, you need to remove 3 push-in clips on the driver's side trunk carpeting. Two are near the brake light. The other one is in a small recess on the side, near the antenna mast. Pull the trim piece back and locate the metal box (looks like a small amp) with wires coming out of it. There is one main plug, about 1.5" wide, and 2 smaller plugs (maybe more, I don't remember, exactly.) The red/pink (or was it pink/red?) wire is on the right side of the main plug. Use a wire splice connector (Radio Shack!) to tap your remote wire from there. I have the power antenna and the MACH audio system, but the wire is probably there for all trim levels.
And to add to that, you can tap your rear speaker wires for the audio signal. I hope you have a line output convertor (converts speaker signal to RCA.) Auto parts stores and car audio places have them, but there are cheaper sources if you have the time to wait for shipping.