floor vents not functioning/opening, HVAC 86' 300E W124
I have an 86' 300E that I purchased nearly six months ago from the original owner. The floor vents fail to open when the correct button is pressed (confirmed by owners manual). I assume they are operated by a vacuum, however I have no idea where to begin my troubleshooting.
Any suggestions or advice would be greatly appreciated.
All other functions of HVAC work great.