Check the vacuum at the two vacuum hoses going into the firewall on the passenger's side. They both have check valves on them. With the engine on, they should both show the same vacuum level on a gauge. With the engine off, if you pull the check valves off the lines going into the firewall, one will make no noise (this is the one going to the dome light sensor) and the other should hiss since you are dumping the vacuum. If this is the case, then your leak is on the dash side of the firewall. The one line goes to the 4 ball vacuum reservoir on the passengers' side, from there, the vacuum goes to the electronically controlled solenoid bank next to the reservoir that then distributes the vacuum to the specific vacuum bellows in the ducting via different colored vacuum hoses based on what your dash knobs are set to.

DEWFPO
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1998 S90 083,228 and 1995 964 154,100