Sparta Ion AliExpress: connecting the battery rearlight (PMU4 70999)

On the Sparta ION battery there is a rearlight that you usually turn on an off via the display. The display tells the BMS to turn on the rearlight. As this Sparta ebike has been converted to an AliExpress type, the on/off switching is a bit different. You need an actual switch so it no longer runs through the BMS. But the rearlight is integrated into the battery so how do we connect the switch and the rearlight? We just use the wiring harness from the battery!