Lim, the rheostst for controlling the instrument cluster and all other controlled light, does not control the lights.
M/B realised that there were too many bulbs(too large a load) for a rheostat of the size fitted on the cluster to control the lights.
Behind the cluster attached to the body is a "solid state voltage controller".
Turnig the dash knob controls th S/S controller.
Modifing the pot will not increase the output of this item.
You run the risk of damaging the controller and that would pop you for around $150.00.
Do as we all have done, squent your eyes, or as I, keep in the center console a small one cell pen light.
Happy Trails Beep Beep from Houston!!!
Donald, El Cheapo