There are four channels for subwoofer. Each can drive 2 Ohms. The 4 Ohm speakers will draw half the current of the 2 Ohm coils, plus there are only two coils not four. The way to get the most out of this amplifier is four 2 Ohm coils. Aside from the original Harman speakers, you can use four single voice coil 2 Ohm speakers, or eight 4 Ohm speakers paralleled in pairs. Just find room for them. :-)

Pin 3 is from the wheel speed sensor to the amplifier. That connection makes the volume go up with vehicle speed. The power top connection changes the equalization in the amplifier to sound the same top up or top down. If you have a coupe it does nothing. Neither has to be connected to anything if you use a different amplifier, but it is important to insulate those wires, prevent them from connecting to anything if you use another amplifier. If you find an amplifier that can use these signals, let us know!

The pinout you posted has a few errors. None of the speaker connections are grounded. This is more accurate:

1 A Radio5
2 A Radio6
3 A Radio6
4 A GND
5 A RTO
6 A Radio4
7 A Radio3
8 A Radio5
9 A Radio4
10 A Radio3
11 A GND
12 A
1 B IGN-ON
2 B
3 B
4 B LTRR2+
5 B RTRR1+
6 B LTRR1+
7 B RTFT+
8 B LTFT+
9 B IGN-ON
10 B
11 B
12 B LTRR2-
13 B RTRR2-
14 B RTRR2+
15 B RTRR1-
16 B LTRR1-
17 B RTFT-
18 B LTFT-