I have a '92 740 wagon and the line to the rear window washer seems to be clogged somewhere between the reservoir under the hood and the rear window outlet. I know that it isn't the filter because I can see that the filter holder is empty and has some fluid in it.I disconnected the hose just above the filter holder and when I blow into it toward the rear window it appears to be clogged. What should I check, where and how?