A fault such as this has to be in a common section of the system. A fault in one controller cannot affect the other, so we are looking at the DCI - but even here, there's little of the circuit that can affect both channels.
Check the ignition switch. If this is left out, the system will be totally dead.
The DCI powers is obtained from the battery via pin 1 of channel 1, thence via the ignition switch. Check that, with the ignition switch closed, you are getting battery voltage on pin 2 of the ignition connector. If not - wiring or switch is wrong or you've blown a fuse track.
If battery voltage is present, check that you've got 9v1 across both pots. The DCI circuit is available from our sister site, 4QD-TEC. The ignition switch turns on a 30mA current source which feeds a 9v1 zener. This circuit is 'fail-safe' in that almost any accident you may have will cause the zener to fail, short circuit, de-powering the board. The 9v zener is the fourth component in the top row of the board, behind Controller 2 (near the mode links, see page 5 of the DCI instructions)
Swap over the two 3 way plugs to which the pots connect. Does the fault follow a pot? Too much heat on the pot terminals can damage it.
Swap over the 6 way connectors Controller 1 and controller 2. Does the fault follow a controller or does it stay on the same output of the DCI?
If it follows a controller, check you've not turned the gain to zero. Swap the motors. Does it follow a motor, or stay on the controller?
System has reverse polarity protection and won't switch on
When reverse polarity protection relays are fitted, you must have an ignition switch. The capacitors in the controllers must charge up before the controllers are turned on or the relays will never pull in. This requires a few seconds delay between connecting the batteries and closing the ignition switch.
Other Faults
There aren't many other problems: if you've reached here the choice is between sending the DCI back to us - see Index: Service, Return and fault finding - and trying to fix it yourself. The circuit is at the 4QD-TEC site - Joystick interface circuits with a description of how it operates.