Getting the Scosche Line Output Converter Wired Correctly
The Scosche LOC-2 or similar line output converter is one of those unglamorous components that makes high-output head units play nice with factory amps. You drop an RCA pair into the speaker wire harness, the LOC turns that high-level signal back down to low-level, and everybody's happy. The problem isn't the concept. The problem is how people actually wire it in real vehicles, and the diagrams you find online often skip the details that matter. Here's how it actually goes together. The LOC has two sets of terminals: high-level input and low-level output. On the input side, you've got plus and minus for left and right channels. Most Scosche units use spade connectors or screw terminals for the high-level side. You tap into the speaker wires at the back of the factory radio or at the amplifier input. The trick is doing it without creating ground loops or losing signal to noise ratio. On the output side, you run shielded RCA cables from the LOC to the factory amp input or to an aftermarket amp if you're bridging the two systems. Ground the LOC chassis to a clean metal point on the vehicle body. Not just any bolt — I learned this the hard way on a 2014 Ford F-150 where the mounting bolt was painted over and had barely any actual metal-to-metal contact. Added a star washer and scraped the paint, and the hum went away completely.
Power the unit if it requires external voltage. Some Scosche LOC models are passive and don't need a separate power feed. Others have an LED indicator and need 12V constant. Check your specific model. The passive ones tap power from the speaker signal itself, which works fine for most applications but can struggle with low-output speakers or very long cable runs.
Where People Go Wrong
The most common mistake I see is connecting the high-level inputs in parallel across the wrong speaker wires. If you tap left channel into the right speaker and vice versa, you'll get phase issues and the stereo image will be completely messed up. Always verify which wire is which before you make the connection. Use a multimeter in continuity mode with the radio disconnected. Trace each speaker wire back to its terminal on the head unit. It takes maybe ten minutes and saves you from tearing everything back apart later. Another issue is grounding the LOC to a point that's already sharing a ground with the factory radio or amp. This creates a ground loop. The symptom is a hum that changes pitch when you rev the engine or turn the AC on. If you're hearing that, move the LOC ground to a different metal point on the chassis, ideally as far away from the radio ground as practical. I've seen this on Subaru Outbacks where the factory ground points are clustered in one area and there's no clean spot nearby. Sometimes the LOC output level is too hot for the receiving amp's input. This causes distortion at higher volumes. Most Scosche units have a fixed gain, so you can't adjust it. The workaround is either adding a line driver with level control between the LOC and the amp, or reducing the head unit's preamp output if it has one. A lot of modern head units have adjustable preamp voltage. Setting it to 2V instead of 4V pairs better with sensitive aftermarket amplifiers.
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Practical Installation Notes
When you're tapping into factory speaker wires, don't cut the wires if you can avoid it. Use tap connectors or solder and heat shrink. Factory wiring gauges are usually thin enough that a poor splice can introduce resistance and degrade signal quality over time. I once worked on a vehicle where the previous installer used Scotch locks on all four speaker taps. Two years later the connections were corroded and the loc was getting an inconsistent signal. Retraining with proper solder joints fixed the issue. Route the RCA cables away from power wires. Running them parallel to a power cable for more than a foot or two invites interference. If you have to cross a power cable, do it at a 90-degree angle. This is basic electrical theory but people routinely ignore it when they're working in tight spaces under dashboards. For the diagram itself, the Scosche LOC-2 wiring diagram shows four input terminals labeled L+, L-, R+, and R-, and two RCA output jacks with ground screws. The physical layout varies slightly between models but the principle stays the same. If you need a reference, the manual on Scosche's website has the official diagram, but it's fairly bare-bones. It won't tell you about the ground loop issues or the speaker wire identification process I described above.
When a Line Output Converter Isn't the Right Solution
There are cases where an LOC is the wrong tool. If your factory system uses a digital communication protocol rather than analog speaker-level signals, the LOC won't help. Some GM and Ford vehicles send audio data over a dedicated serial bus, and tapping speaker wires gives you silence or noise instead of a usable signal. In those situations you need a protocol-specific interface module, not a line output converter. The Toyota Camry hybrid is a classic example where a simple LOC doesn't work. Another limitation is dynamic range. Factory speaker-level signals can be quite loud, sometimes 20 watts or more per channel. The LOC converts this down, but the signal-to-noise ratio of the conversion isn't perfect. If you're building a quiet system with sub-ohm loads and a high-gain amplifier, you might notice a slight increase in background noise. It's usually subtle, but audible in a well-built system. If you're starting from scratch and the vehicle doesn't have a factory amp, consider a head unit with built-in preamp outputs. That eliminates the LOC entirely and usually gives cleaner results. But if you're working with a factory system that already has an amplifier and you need to add an aftermarket component, the LOC is still the standard approach. It's not glamorous, it doesn't make things sound dramatically better on its own, but it makes the whole setup possible. That's what these things do.
