The Soundgate LOC4.2 passive line converter (converts speaker output to rca output) seems to work quite well. I do not have experience with other brands, but the installation was easy (although professional install is recommended). I definitely did not notice any profound drop in sound quality from my experience with other rca-level input. Converting from speaker level can lead to problems if it's not done right, but with this one it seems you cannot go wrong. If you find noise in the system a possible step might be to ground the radio close together to the grounding point of unit receiving the rca input (amp, processor, whatever). I have not found this necessary so far.
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