Quick answer
Hiss or a weak signal into your downstream amps usually means too many cheap conversions, or you've tapped the wrong point. Use your headunit's real pre-outs if it has them. If it doesn't, use a quality LOC or a DSP high-level input and keep the path as short as you can.
Problem
- You're choosing between a line-level (RCA) feed and a speaker-level feed into your amp or DSP.
Symptoms
- You get hiss or a weak signal.
Cause
- Your factory radio has no RCA pre-outs.
Fix Step-by-step
- Audit the headunit: check for dedicated front, rear and sub RCAs. If they're there and you're not stuck in a factory DSP chain, plan RCA → amp/DSP first.
- No RCAs, or the factory radio stays: pick a quality LOC, an amp high-level input, or a DSP with a rated high-level input, not twisted speaker wire jammed into an RCA.
- Retained factory amplifier: tap only where the integration sheet allows. The wrong tap gives you hiss or an amp mute. Match the turn-on and voltage expectations.
- Set the amp input switch: LINE or HI, whichever your brand labels it. Common mistake: leaving a high-level feed going into a line jack, which you'll hear as weak or distorted straight away.
- Keep the hops down: radio → DSP → amp beats radio → budget LOC → second LOC → amp.
- Gains last: set minimum gains, pick a sane headunit baseline volume, then follow the gain guide. On a high-level path you hear the noise floor sooner.
Real-world gotchas
- If you ground an LOC under the dash while the amp grounds at the tail, you get ground-loop hiss.
- Feed RCA and high-level into the same channel just to test it and you'll often damage the input stages.