Easy way to do it: Just get some RCA T connectors and plug them into the input.
However I suggest you use the speaker outputs to drive the plate amp but not through the high level inputs as the impedance is low and will mess up the net impedance seen by the amp. Instead use a ~1K resistor in series and connect to the low level input which has a very high input impedance and will not affect the net impedance. Then the sub gets the same signal the speakers get, albeit attenuated to match levels.
Credit for the above goes to our absent friend, Thorsten.
Brian
Take control! DIY.