I'm back with my dual 3.5" speaker brackets

Capacitors in the speaker circuit which prevent over driving the small speakers. Kind of like a basic crossover network. They keep the small speakers from trying to create too deep of frequency and buzzing or popping due to lack of control. You can use:
496 MFD capacitor - blocks below 80Hz @ 4 Ohms
398 MFD capacitor -blocks below 100 Hz @ 4 ohms
330 MFD capacitor -blocks below 120 Hz@ 4 ohms
This is an approximation so capacitor values can be slightly different.

These can be sourced from your local car stereo shop (probably expensive there)or a place like Parts Express.