What you didn't tell us is what kind of vehicle this is. Many, and I mean many, late model vehicles have all kinds of challenges facing you in the digital domain that must be undone or normalized before installing new gear - equalization contours designed to make $3 speakers sound like $30 ones, fully active systems with each component having a narrow bandwidth of operation and / or delay, multiple position contours overlaid on top one another, etc. In addition, you could have some impedance mis-matching with your interface which could cause frequency response irregularities, etc.
So, share a bit more.