I really need to get through the firewall

So I finally started installing an amp and sub in my W123 '84 300D. I have everything run up to the dash/console area, but I can't find a way to get a power wire straight from the battery back to the amp.
I have a 10 gauge power wire run to the front, but I don't know where/ how to get through the firewall and into the engine compartment. I ran along the passenger side as someone recommended, but I don't see anywhere to get into the engine area near the battery. The only option right now is on the drivers side where the vacuum lines go in, but I can't find where they come through on the inside. Any thoughts?
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There should be the same on passenger side... by the battery... take the battery out... and any cover behind the battery.
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Another thing you can try is coming in by those vacuum lines and runnign under the dash. getting to the spot int eh firewall where those vacuum lines come through is pretty easy. Just pull out the gauge cluster. Carefully pry around the edges until you get it loose, then pull it out. Take off what you need to to set it aside. (Be careful of the oil filled oil pressure line.) I'd prefer this method simply because I don't like climbing under the dash and working on my back. Unfortunately you will still need to get the wire over to the other side of the dash if you use this hole.
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This was my original thought since I didn't want to pull that huge battery out. Also, getting those plastic pieces off the bottom of the dash is pain. I've tried twice this summer, but gave up because of the heat (its been around 100F here for awhile now -- Yes, I know its hotter out west, but I'm sweating here). My thought was that it would be easier to get in by those vacuum lines and in behind the instrument cluster (I've pulled that out before). Getting down across the other stuff won't be so bad since I'll have the radio and climate control stuff out of the center anyway (to hook up the stereo stuff). I'll see how bad that battery is to get out first though. Shorter wire is preferred, especially with this direct power line.
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