Pull the rear feed hose off of the pump and actuate it. If you get a squirt then the pump is OK. Believe the pump is located at base of resevoir. Another thing to try is blowing some air through the hose from the pump end and getting someone to listen at the rear. If the passage of air can be detected, then the line is at least partially open. Get a small hand operated vacuum pump, connect it to the pump end of the hose and have someone occlude the washer end. Try to establish a low level vacuum and see if it holds. If it does then you probably have no leaks. If not, then there is probably a hole somewhere. Most of the time these hoses are routed along a channel in the body or along a frame rail and up the rear pillar. You'll have to try and follow it from the engine compartment to where it is no longer visible which may tell you intuitively where it is routed. Good luck..

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Jeff L
1987 300e
1989 300e
1987 BMW 325