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I have a 92 chevy truck. I got in yesterday and the heater window was black… It does not have a stock radio in it and hasn’t for over 10 years. I figure it’s that control module in the dash. I go to the junkyard and ask the man there if he’ll allow me to plug in the controller he is about to sell me. I did, and it was black also. I go back in and say it’s not the problem. He goes into the back and comes out with a radio. He says the heater goes thru the radio…?? He says this unit is under the dash. I tear the entire dash out and can’t find it. I trace the wires from my controller to a series of relays…I run direct power to the controller and it’s still black BUT, the temp control slider is backlit. HELP ME!

I conclude this is a full size truck? The radio in your truck is refered to as a “three piece system” and yes the fan speed display is part of the radio display.

The three pieces are the control head the reciever and the external cassette. The control head and the reciever must be compatible things will work without the cassette pluged in.

Try finding a good reciever/ control head pair.
The reciever does not look like a radio reciever as it is never seen,it looks like a metal box with wires pluged into it (the antenna is the clue)
I can change all three parts in under 45min,the tearing out the dash part worries me.

The running direct power route never turns out well,fix it as designed is always the best approach. Not everything works on +12V.

Miser Guy, Have You Checked For Technical Service Bulletins? There Is At Least One TSB.

1990-1993 trucks (C/K models) have control heads that may not illuminate, especially after replacing the head or disconnecting the battery, but it can happen after any ignition cycle.

the condition stems from the control head not receiving a correct signal when the ignition is turned on.

First try turning the ignition off and on.

Next disconnect and reconnect the battery making sure that a good electrical connection is maintained while reconnecting it. Avoid “chattering” the battery line.

Thank you oldschool and common sense answer. I spent the day checking for voltages and shorts. After 4 hours and $63.00 in replacements parts that did not fix the problem, I found the system had lost it’s ground. re-grounded it and it worked fine…