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I didn't specifically try to set the brake and play with it, but being a manual tranny I use the parking brake all the time and I fiddle with the **** from time to time and it doesn't seem to change the idle, but today when I go to lunch I will give it a try......
If its not OEM pull the panel and follow the wires if there is any. Could be something the PO put in it then removed when sold. Its not your instrument panel light fader is it?
Finally got around to trying the idle up theory, it's not that, I guess I'll have to stuff my big *** under the dash to see if it's hooked to anything and where it may lead to.......
That **** most likely adjust the brightness of the dash cluster lights. Some appears to have added the white gauge faces to your cluster. At night with lights on, try turning the ****.