Display is very dim in the daytime.impossible to see anything

The dimmer switch only works when the headlights are on, When they are off there is a light sensor on the dash that takes over and adjusts the dash lights as needed. It sounds like yours has gone out.

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Daytime headlight issue ? No , no sensor behind the front bumper . No Daytime time head lamp issue , doesn't turn on the instrument panel light's . This is more then likely a remote dimmer module problem .
Interior Lights Dimming Circuit Description (Pontiac)
When the park lights are on, battery positive voltage (B+) is applied via CKT 32 (YEL) to the remote dimmer module. The interior dimming switch in the lamp dimmer switch is a variable resistor which applies the dimming input via CKT 44 (DK GRN) to the remote dimmer module. The potentiometer in the lamp dimmer switch cannot provide enough current in order to operate the incandescent lights. The remote dimmer module amplifies the current, applying a variable voltage to the incandescent lights. As the amount of current increases, the lamps become brighter. As the amount of current decreases, the lamps become dimmer.
The radio, heater and A/C control assembly, instrument cluster, and passenger climate control each has a vacuum fluorescent (VF) display. When each module receives an input that the ignition switch is turned to the RUN position, each module activates the VF display at the full intensity. When the park lights are on, the radio receives the dimming level from an input on CKT 8 (GRY). The radio then relays this information to any other modules with VF displays. All modules with VF displays then adjust the dimming level of their VF display in order to match the level of the incandescent lamps.

That is a daylight sensor on the dash and when covered it actually will dim the display for night time driving.

There is nothign wrong with your car-when you turn on the headlights the car believes you are turnung them on because it is dark outside, so it automatically dims your dashboard lights for you (since it now thinks it's nighttime). Your car already has daytime running lights, so you don't need to turn your headlights on in the daytime-they come on automatically when the car is running. You can keep your dash lights dim or bright by adjusting the small black dial on the left side of the dashboard, whether headlights are on or off if you want to.

this is because the screen has a ribbon on the glass to the board over time in hot and cold weather it loses is connection, you can replace the ribbon and it will work fine after that but it will do this again later one as its inevitable. if you wish to change the ribbon you should be able to get one from this site. Home Page

there is wires that control these funtions under the steering column. i would take it back to them and have them at least recheck their work. if they don't, then take it to an electrical shop and explain to them about your situation.

u definately have a nake wire thats causing this scenario. U can start tracing it from your stop-light switch into main power supply loom. Make sure u tape any naked wires and replace broken fuses. Good luck.

i would say yor problem is most likely corrosion of the connectors in the engine compartment to the light assemblies. this would affect everything you have mentioned

what happens when you turn the knob to brite/dim the dash lights?

Look for the relay for the head lights,it may be bad.Also look for a bad ground.And check the bulbs too.