I put in a stereo last night but I wasn't able to find a good place to get power through the firewall so I could power the new amplifier (it's in the trunk). I found +12 and +12 switched in the trunk but it's not really working.

So, what's the best way to get through the firewall so I can hook up the amplifier correctly on an '88 300E?

On the W124 there are grommeted entries through the firewall to the inside of the car behind the battery where the ECU's are. I don't have any experience with the sedan as to where you can get from the rear interior to the trunk.
If you are looking for a place to connect to 12+v, try the terminal just in front of the battery where the alternator/starter/battery possitive cables connect - there are two additional connectors there. You may want to run a high amp. line to the trunk in case you need more power in the future. Don't forget to fuse the feed near the terminal.

I have power to my rear amp/subwoofers... through the passenger side back seat area.....

runs through the car...comes out near the battery...and then has a separate fuse box hooked up to the battery terminal

if you have the 6 speaker stereo there should already be anamp inthe trunk. in which case why not remove the merc amp and use its power supply?

of courxe if you have the standard stereo you will have to route a power line through the grommets on the firewall. theres one behind the rear seat as well to give you access to the trunk.

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bmw 2002.
long live the legend

To get the wire from the cabin into the trunk, you'll more than likely have to pull out the rear seat back and cushion to access the grommets.

The cushion comes out first, push in on the red tabs and lift the front, then pull out. It's pretty light, but watch your fingers, the metal frame is sharp. The seatback is a bit harder, there are a couple-three bolts on the bottom that unscrew, then the seatback lifts up a couple inches and then comes out forward. This one's a pain because it weighs more and it's hard to get leverage when you're hunched over in the cabin. Then you can pull up the insulation and find the wire tracks up to the grommets in the upper part of the firewall.

If you're willing to remove the front seats too, you can use the factory wire channels. If not, you can fit a couple wires under the right and left side door sills, they pop up and the trim along the way can be loosened by undoing the screws.