На усях Alpine серии MRD (M300 и М500) ставился выносной резистор 5 кОм:

---
Connect a 5k potentiometer between pins 2 and 6. 5k=Full Volume,
0 Ohm =Minimum Volume Note the odd pinout of a 7 pin DIN connector.
Facing the amp, clockwise starting on the bottom left: 6,1,4,2,5,3,7 Pot connects to bottom left and top pins.
Pin
Number		 Pin
Name		 Description
1 Batt Unused
2 Bass Adjust 5K Pot to Gnd
3 CMD CLK Unused
4 Commander Knob Unused
5 CMD IN Unused
6 GND 5K Pot to Bass Adjust
7 CMD OUT Unused

https://pinoutguide.com/Audio-Video-...ass_knob.shtml