I saw that reference to ground switching which I found a bit confusing. I am wondering whether it was related to the fact that on modern cars, certainly Japanese cars, the practise is to have the +12v side of the device permanently connected and then control by switching the ground connection on the bulbs. Implementing the dimmer on this type of arrangement would be more complex because the dimmer needs a switched supply on its source and the original 12v 'source' to the bulbs needs to be isolated from the bulbs.

The 'other' dimmer was a three wire device. I expect: 1) +12 v source, 2) common ground, 3) PWM output to the LEDs. In such case, the dimmer ground would be connected to chassis ground. The + terminal of all the LEDs (assuming they are polarity sensitive) could be commoned and connected to the dimmer output and I presume that the individual gauges lights already use chassis ground? The dimmer with the 4 terminals can probably be set up just the same by connecting the two ground terminals together and then connecting to chassis ground. Connect the dimmer input to switched 12 v power and the output directly to the LEDs.