there's a factory amp. Did you plug the amp remote on / antenna on wire to the appropriate one in the harness?

You can go speaker level into it and it works fine.

Do that.

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)