How should I wire a dimmer into a 12v DC SPDT On/On setup
The section or graphic titled 'SPDT ON/ON SWITCHES' near the end of the linked article illustrates exactly the same circuit used for a "3 way" light in a mains-powered home.

If you want the dimming function to happen regardless of which switch is turned on, that's easy. Insert the dimmer in the connection between the battery and first switch, or between the second switch and the lamps.

While technically it would work to let the switches cut the power from the dimmer to the lamps (ie, insert the dimmer between the battery and the switches), it's probably superior to place the dimmer between the switches and the lamps. With it placed here the switches cut the input power to the dimmer. If the dimmer were left always powered as in the first arrangement then it would consume some energy even when the lamps are disconnected. That's not ideal in a battery-operated context.