Cut the bus bar. Divide it as you need. Solder a wire or stud to the cut off section, and power it from one of the fused side circuits.
There is a potential problem with doing this though. Some relays have an internally configured suppression diode. Since you will be firing the coil from a reversed polarity standpoint from which it was designed, the suppression diode wont do it’s job. (if it is so equipped)and will short circuit. Using a non suppressed relay will allow doing this.
The diode is there to prevent flyback voltage from making its way back to the control unit. You could easily add suppression diodes to the bar itself in the required polarity.
Google “suppression diode in relay” for more detail on why/ what/ how.