Mercury Repair Questions? Ask a Mechanic for Answers ASAP
Your problem is a vacuum leak. Defrost is the default postion(no vacuum).Start first by finding the small black vacuum line going thru the firewall near the heater hoses to the heater core. Trace it back to the vacuum reservior located near the relay module and from there to the intake manifold and inspect for leaks.Here or pics of the reservior and how to replace it if you find it leaking. If all of this is good then one of the hoses inside from the control panel to one of the vacuum door motors or one of the motors themself are leaking.
1.Remove the relay module
2.Disconnect the vacuum hose
3.From inside the LH wheel well, remove the nuts
Yes it still does use vacuum even on the electronic system just the blend door is electronic.
Control System Outputs-EATC
A/C Blower Motor Speed Control
The A/C blower motor speed control is located on the A/C evaporator housing, in the engine compartment.
A/C Electronic Blend Door Actuator
The A/C electronic blend door actuator is located on the top of the plenum housing.
Vacuum Control Motors
Vacuum control motors are used to move the floor/panel and panel/defrost doors.