When the head unit starts to drop/blink in and out usually there is a bad connection between the main PCB card to control head. Good news you might still be able to fix it yourself. Here are some repairs that I have performed in the past myself maybe they will help.
There are a few options that I know of for getting the C68 unit back on its feet again:
Buy a "new" control unit for ~ $500 (GM stealership)
Buy a "good used" control unit for ~ $200 (salvage yard/Fleabay)
Fix your "old" unit yourself (~free -- if own a soldering iron already)
The C68 control heads have a problematic connector which is the main connection point for the biger PCB contact strip....the finger contacts inside big (white) connector is where your problem is most likely to be found. The unit directly below has been removed from the housing but is unmodified.
Unit breakdown/repair: It is always a good idea to use a ground strap or ground work station while working on electronic devices this is done to prevent damage from static electricity.
Remove the 2 clips from the sides of unit (leave the faceplate connected at this point)by prying them off with a small screwdriver.
Now slightly pull the plastic tabs (black) away on each side of the case while pushing the (green) connector in the direction of the faceplate and the main PCB card will slide out the front of the case.
Seperate the head from the main PCB card pull the card away from the back of control head.
Now for the repair:
You have a couple methods which you can choose from. Choose the method you feel most comfortable with.
Clean the contact points on the PCB card with an appropriate cleaner or pencil eraser (see pic below)
Tin each contact point on the PCB card using a minimal amont of solder (see pic below). The added solder will provide a little extra pressure at the contact points.
Remove the (white) connector cover from head unit and solder the connectors directly to the PCB card. Note: before removing/cutting off the white connector cover measure the gap between the PCB and the head's PCB. You will need to reproduce this gap before you solder the connector to the PCB card...the gap allows the unit's back green connector to be exposed enough to that you can plug the harness connector back into the unit.
The white connector cover can be remove with little trouble if you attack it at each end it is held in place by black plastic pins. Now the contact fingers can be soldered to the PCB directly see pics below. Position the connector and card where you want them to be and heat up the connector pins and flow some solder between them to form a joint. Note: There are 2 different units pictured here one is an '88 (all stock no mods) and one is and '89 - the unit without the white clip is soldered to the PCB card this unit is an '89. This '89 unit has had no issues what-so-ever and performs as it should since my origional repair over 8 years ago.
Final thought....if you have the unit broken down to this level go ahead and replace the 3 light bulbs for the back lighting. Reassemble the unit.
Don't worry if all the button pieces fell out when you to took the face plate off you can just put all the parts back in there and it will be fine.....the clips on the sides of the unit will hold all the loose stuff in place.
Shoot me a PM if you have questions.