This is what I would do if I’m dedicated enough to this laziness and didn’t care about starting a fire in my house:

• The switching voltage is 12V I assume from the datasheet and model number. I’d measure the voltage of the relay (the biggest black box) on the lower side with a multimeter when on if I want to be sure.
• I’d disconnect the positive end of the low side of the relay.
• I’d attach it to a SPDT switch. Middle pin to the relay, side pin to the previous connection on the circuit board.
• The other side I’d connect a 12V input or whatever was measured. I’d use 8AA batteries or a 12V power supply, and 1k ohm resistor connected in series, and if that didn’t work I’d try a couple other resistance values.
• So the switch would be normal operation in one position, and then always off or on in the other depending if my power supply is functioning.