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Tapping power in trunk for amp if you don't have a trunk mounted battery
I have been searching for a simple, non invasive way to tap power in the trunk area for those of us without trunk mounted batteries, and I though I would share in case anyone else is having the same issue.
I initially was planning to run a 6 gauge cable from the front to the back, but routing around the exhaust was proving difficult.
SO I started looking at the trunk fuse panels, and realized I could just plug into the unfused bar for a circuit that wasn't an installed option on my car I simply plugged a male spade terminal crimp connector into the slot for the unused fuse. I decided to use a large circuit breaker as an attachment point under the truck because I had it in my shop, this was probably unnecessary but I was designing in my head as I was installing so bear with me. I found a nice unused ground point that even had the nut already installed and bright metal contact point, nice!
My amp and Lc2i line converter were already installed on the box from my previous car, so I used quick splice connectors to pull signal from the rear deck speakers and ran that to the Lc2i. Power bar is running down on side of all the fuse slots, even the unused ones. made sense at the time... I was very happy to find that unused factory ground point on the passenger side of the trunk! I assume this would be the battery ground point in versions with the rear battery. Finally, some bass.
Last edited by bernard farquar; Aug 25, 2025 at 12:42 PM.
Reason: trying to fix broken images
You may want to keep R-SAM delicate circuitry out of feeding your Amp high-current surges... Tap the cable directly feeding SAM in the back of it.
You already have adequate protection from your nearby 120A fuse and a nice short GND cable.
Needless to say you'd be better off with a dedicated homerun cable to the power source (ALT or Prefuse or BATT) and extra engine-chassis GND straps.
My first thought was to run a cable from front to back, but routing around the exhaust was looking problematic. Here I am making sure the 6 gauge cable I had was long enough . I figured since the panel I tapped into had 40 and 50 amp mega fuses above, and 30 amp fuses directly adjacent to where I tapped, I was unlikely to be drawing too much with my 250 watt mono amp. This is the first Mercedes where I had to find a power source for my trunk mounted amp that wasn't the battery.
120 amps breaker you are using, taking from rear sam not so high power fuse source ( fuse 40 is where you took power, it is labeled as spare ) is a reckless practice #1, because 120 amps breaker you use.
Fuse/breaker must be as close as possible to the power source, yours is at least 2+ feet away. Another reckless point #2.
You must be an audiophile person judging by all those work . It looks like your trunk lost some space but W212s just like many Mercedes-Benz models have a useful trunk capacity .
Happy Driving
S-Prihadi makes a valid point about the circuit breaker. Your 250 watt amplifier, at permanent hearing loss volume, probably draws about 25 amperes maximum. Using #6 cable should be adequate as it's current capacity is about 38 amperes. However, a 120 ampere circuit breaker will not protect the #6 cable. A circuit breaker rated at 30 - 35 amperes would be a better choice.
Another vote for a smaller fuse or circuit breaker at the fuse box, not 2' away from it.
You want a fuse that will protect the wire assuming your amp has a fuse. If your amp has a 30 amp fuse, put a 30 amp fuse on the wire. Something smaller than the wire.
120 amps breaker you are using, taking from rear sam not so high power fuse source ( fuse 40 is where you took power, it is labeled as spare ) is a reckless practice #1, because 120 amps breaker you use.
Fuse/breaker must be as close as possible to the power source, yours is at least 2+ feet away. Another reckless point #2.
Rear SAM gets its circuit 30g power from F32 prefuse box at 160 amps.
Don't let your car have short circuit and caught on fire , please.
There is a fuse on the power cable, I was just using the breaker as an attachment point because I had it laying around my shop. I am planning to add a fuse at the point it comes out of the panel.
Last edited by bernard farquar; Aug 28, 2025 at 06:28 PM.
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