how to: equalize bass tone

I am not familiar with your specific car. However I have dealt with hundreds if not thousands of GM vehicles. GM often uses fake bass technologies. This is not as bad as it sounds. It’s actually quite smart. What they do is boost the bass at 50hz and then cut off everything below that point. This fools your brain into thinking that there is lower bass than there is, and it actually sounds quite pleasant. With this kind of technology its possible to make small speakers (4x10, 6x9, 6.5) produce the sound of large subwoofers. The problem is that when you tap into this modified signal, and apply that to real subs you don’t get a smooth sound. This is due to some complex issues involving larger diameter cones including harmonic distortion, radial distortion, and the fact that this type of brain trickery has its limitations. It only works in moderation. Long story short. If you have taped into a speaker(s) in the factory system that utilizes this technology you will encounter the one note wonder. You need to find an output on the factory amp that has a flat frequency response. Not a modified one. On GM trucks with Bose for instance the tweeters located in the pillars are the only speakers in the system that receives an unaltered signal. This is because the tweeter uses a passive crossover mounted to the speaker its self to eliminate low frequencies. All other speakers in the system are regulated by active crossovers built into the factory amp, there fore unusable. As I said this is only an educated theory based on experience(not specific). If you think that the problem might be something ells, please post more specific information including where you taped your signal from position of your sub in the car and the settings on your amp.