For Nissan cars, the signal of OEM headunit is preamp. Otherwise you better connect the preamps to the speaker cables and you are done, sound will be better. BOSE amplifiers respond really well when they are connected to 4 or 5 Volt preouts. It is all you need. If your aftermarket radio has no preamp output, you need this item. I need the preamps and my AVH-5050dvd has only two preamps. I wired the front preamp to the front speakers (yes, direct) with RCA's, the rear preout to SubWoffer. For the rear i used this unit, adjust the level to the appropriate audio gain (I had a hard time with this, but once made, you are done).
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