Where to connect a equalizer to a 6 output car stereo?

Not to the outputs. A band-pass equalizer works on input signals. To put it simply: if your stereo is the source of the sound, you would have to open the stereo up with schematics of its electronics in hand and find a suitable place to "pop in" an equalizer. If your stereo is only taking a signal from some other device and amplifying it (in essence doing the job of an amplifier with external signal source), put the equalizer between the source and the stereo.

Please note that an antenna is NOT an "external source" - the radio frequencies are decoded INSIDE the stereo, and only then produce a valid "input signal" - all within the electronics of the stereo.