I'd pull your light switch and tap into the dimmer line rather than trying to go into the cluster. There should only be 4 lines running in/out of the back of the switch, so get a volt meter and check for passing voltage in each line as you lower the dimmer intensity to determine which line you need.

 

I'd email autometer. They'll tell you within a day. They sent me a beautiful graph on my Pyrometer gauge on the diesel for wiring.

 

 

LEDs typically don't "dim"... they are either on, or off. Just tap into any "courtesy light positive wire" circuit for the gauge to follow the dash lights on/off.