1998 olds 88 heater off switch is on a vacuum line

check the instrument cluster fuse.make sure instrument panel brightness control knob not turn off that will cause no dash lights.

do you mean the fan or the temp selector, , ?If temp gauge/selector go under the dash on top of heater core box or on the side will be a lever on older cars it was cable operated newer cars vacuum, try moving the lever by hand, vacuum line may be broke or module may be bad, thats why its not closing .

I think you are correct , the audio store must of knocked off a vacuum line or a plug at the heating controls, this being a 1998 and conversion it may have some vacuum operated components and some electrically operated actutor motors that control the doors for the heat and the selector. i would start off by looking at the controls to see if everything is connected good,i think you will find something they broke or left off. the radio was replaced then i would have them remove it and double check there work,let me see if i can find a diagram for you, okay heres how it goes, the blend door which controls the hot or cold air is electrically controled with an actuator motor, the selector control to send air to the defrost or vents or floor is controlled by vacuum, theres an 8 port vacuum switch on the heater control that operates the selection, the vacuum line is brkoen or they broke or left off the 8port plug that sends vacuum to operate the valves to control the flow of the air. theres a main vacuum line that runs from the intake manifold in the engine compartment and this line goes in on the pass/side into the passenger compartment and then up the dash to the controls , i would start at the controls and make sure theres vacuum to the control and the 8port plug is plugged in, also check the engine to make sure the main vacuum line is connected, you would hear a hissing noise if removed, also start the engine with heater fan off and listen closely for a hissing inside the dash area by controls, if you hear this then it would mean theres a vaucuum line loose or off, good luck hope this helps. heres a diagram of the vacuum lines for the heater control.

It might not be a fuse that goes to that it might be a relay thats under your hood in the fuse box. Just a idea for you to try. Hope this works out for you.

You don't say whether the fan starts and shuts off with the bumps, or the heat goes cold but the fan stays on. If it's the fan shutting off and coming back, you most likely have an intermittent connection to the blower motor somewhere at the fuse box, the fan speed control switch or the fan motor, or a loose ground connection at the fan motor. The fan motor is visible under the right side speaker grille on the dash. Make sure you re-connect the short black corrugated hose to the grill vent or you won't have defrosting on that side!

The air distribution rotary switch controls where the air goes. The fan speed switch controls exactly that: fan speed. the temp rotary switch controls the coolant temp into the heater core. You need to clarify what you mean by "Also, the dash controls for the vents won't allow me to turn up the power"

You might also have one or more of the following issues:

1. is you vacuum tank properly connected and are there no leaks in the vacuum piping? The tank is in the driver side rear wheel well below the trunk deck on a convertible, and in the passenger side front fender area on the other cars. It's a white bottle with one or two hoses connected to it. If not properly on, your vacuum operated heater flaps won't move. Does the air distribution knob make sucking or hissing noises when you set it to different positions? If not, check the tank connections, look for vacuum line leaks, then suspect the vacuum valve behind the distribution knob.
2. if the air distribution rotary switch makes noises when you change its setting, but nothing happens, then suspect the vacuum solenoids that open and close the heater flaps - could be a loose vacuum line or a defective vacuum solenoid
   
3. the shaft connecting the temp control knob to the heater valve behind it may not be properly in place. This can be inspected, and adjusted if necessary, from the left side speaker/vent cutout on top of the dash. Make sure you re-connect the black flex hose to the air vent tube, or you'll get no defrosting afterwards!
4. remember, the centre vents always blow cold air; a brilliant idea from Saab so the driver (and passenger) can get fresh air in the face and stay awake while the rest of the driver, passenger and car is toasty!

Look for a vacuum line passenger side rear under hood near hood hinge area. Follow this line back to the intake manifold. checking for crack and leaks if any breaks in this line repair them this should fix your problem if not check all vacuum lines under the dash as well as the vacuum diaphrams that control heater door operations. if you have a hand vacuum pump you can check all these at (previous found) intake connection for the heat controls

The most common problem that causes this is a bad temp control, the actual selector module in the dash of the car. Try replacing this.

on the right side of steering colum there is a plug. there is a wire that gets warm and loses connection. check by moving plug. my heater and ac would not work and caused the vacume swith to stay open.

You have lost engine vacuum to the AC vacuum motors, this is either a pinched vacuum line from the engine vacuum storage canister or the canister is damaged or the control switch in the dash is leaking vacuum.