How to lower the bass in my car?

I have an amp controlling a subwoofer, and I want to lower the bass. When I play music through my ipod I can set the equalizer to "bass reducer". When I use amazon music/pandora with my iphone I don't have that option, so what can I do? I already have the bass level all the way down on my car receiver. Is there an in-line equalizer I can hook up to the iphone cord?

MOST automotive stereo systems can adjust the bass response through the speakers. When in doubt, try actually READING the owner's manual.

First, we have to agree on exactly WHAT you want, as the word 'lower' can have different meanings. You want to 'lower' the bass, like get the frequency response of the bass speaker to play lower frequency notes? Or like to reduce the power output of just the lower frequency notes it is already playing, to make just them quieter?

Things you might be able to do:

Look for "bass boost" on your amplifier and set it at its lowest setting, or at least a lower one.

If your amp has a subsonic filter, turn it on.

If your amp has a crossover filter, lower the frequency so the subs play less of the higher ones.

If your amp has a remote subwoofer level control, turn it down

If your head unit has tone or EQ settings in the various levels of menu's, lower the gain on bass frequencies.

If you have ported sub enclosures, switch to smaller sealed enclosures. If your subs are in large sealed enclosures, but them in the smallest ones recommended by the sub maker. If you have ported enclosures, alter the port length (if possible) to change the resonant frequency.

Buy an equalizer.

And, as Doc says, READ THE MANUALS!

None of my speakers have any bass coming out of them. It all comes from the subwoofer. I believe the installer said my factory head unit was not able to separate speaker bass from sub bass, so the head unit adjusts just the sub. The bass level is all the way down like I said. I want to REDUCE the bass. Its fine when using ipod touch because it has an equalizer. I can't adjust bass through pandora

Just get yourself a parametric equalizer like boschmann. That will do all the tricks

Try using an attenuating patch cord to play from the player to the aux input.

I think we're over complicating this.
Look at your amps controls. There may be a bass boost setting. If it's not at zero set it to zero. That may be the issue. If it's already at zero look for the gain (or level or sens.). Reduce it by 10-20 degrees and that will reduce the loudness of the sub.