Factory stereo with external amp

The factory stereo on my 96 Explorer was only a cassete player so I want to replace it with another factory stereo from a 98 Explorer that has both a cassete and CD player they both have the wire for an external speaker amp but the CD player one works without it connected, and there's no difference in sound level when I connect that wire, do I need it? Also volume from left side is lower than that of the right side is this normal or am I doing something wrong I have checked the speaker connections many times already.

The wire connection guide shows only one cable for grounding the external amp and it is in the same harness as the batt V, constant V and ground for CD player. I did try the balance and fade on the CD player trying each speaker individually that is how I noticed the difference, they're all the same age.