It’ll be best just to get the combo switch rebuilt as well. There’s a guy named GD Humfries whose a master at rebuilding those switches. But like mike said, it could also be a ground issue. Also check the connection to the dimmer switch. I had the same issue once when the dash lights didn’t turn on. Discovered that it was the connection on the back of the dimmer switch. Connected and disconnected it several times to get a good ground and connection and BAM, everything worked.
The photo is of my spare 85/86 switches that Were rebuilt and tested and work like new. Keep in mind that the switches for the 83/84 have a different connection to that of the 85/86. If you plan to purchase a used rebuilt one, Make sure you are getting the correct one.