Yes the plug was on the back of the head unit. When I had my radio installed, they had to pull the head unit out and when they put it back in, they didn't plug everything back in properly.

 

Letsgowheelin... do i need to take my dash apart to get to these plugs?

joe

 

I have a 1997 F150 and I solved my problem.
There is a check valve between the battery and firewall. Mine was stuck under the battery cover and acid had eaten a hose and also corroded the valve. I went to Advance Auto Parts and bought a new check valve and it solved my problem. The check valve is what keeps the pressure constant when accelerating.

 

Actually, it's the combination of that check valve and the reservoir in the fender that keeps the vacuum to the control head from dropping during low intake manifold vacuum conditions.

Steve

 

My 2003 Supercrew Lariat was alternating between dash vents and defroster vents.......then I had the battery replaced and now the air will only come out of the defrost vents. Do you have any pictures of where this "check valve" and "reservoir" are located?