I was working on my 98 subaru legacy today and I was about to put the cover on the throttle body, when I noticed a small vacuum line was disconnected. This vacuum line comes from the pressure exchange solenoid and one end connects to the fuel pressure regulator and goes thru a small filter, but what does the other end connect to - and where is it located? 

Not sure what you mean by pressure exchange solenoid.

There is a pressure source switching solenoid on the passenger strut tower. And the Evap purge solenoid under the #3 intake runner.

 

The line with the filter is supposed to go to the switching solenoid, which has a small line that then goes to the MAP sensor right next to it.

finally found where the vacuum line goes - it goes to the intake manifold just below where the vacuum line from the power brake booster connects to the intake manifold