Ok, so yourv got the ACC 12V power connected to your gauge and your looking where to plug the illumination of the gauge so it can dim with the dash factory lighting?
I think it can be done on most case without splicing into the factory harness, just check the fuse panel there is some tab on the fuse panel.
You have, Bat, Ign, Acc and lps you can have a connector there and call it done. Gauge illumination will work same as the factory dash lighting.
Am going to seek for some picture about what i mean. brb(hopefully) with some picture...
EDIT
k i found some picture..
Pardon me because idont have my car with me right now.
anyway see the fuse panel picture, you can see the connector i was refering to.
Now on the schematic(that is for a older model but thats alright for us right now) you can see the LPS tab connector.. usually from the dimmer switch for the gauge illumination. It has the "20 GRY" wire after the lps fuse, thats where you want to take your gauge lighting from.
So just look for the LPS connector(grey wire usually)on the fuse panel, if the writting is gone you can use a multimeter to find the right one, if there is none just splice into one of the dash light positive grey wire.
The grey wire with the green connector in the picture should be the lps wire if am right. the picture is not from my car.
Hope it help.