Here’s what I would suggest.
Make several copies of the schematic and get a hi-lighter pen. For each position of the stalk switch, hi-light the circuit from the ign +12v (LG) to motor ground.
Let’s do it for Slow first. Current goes to SW-1 (switch pin 1), SW-5, M-5 (motor pin 5), M-1, SW-7, SW-2, ground. Motor runs in the normal operational direction.
Strangely, the Off position is more complicated, since there are really two states: when the Park microswitch is in position Z (parked) and when it is in position Y. When first switch to Off chances are high that the motor is not in the parked position so the microswitch is in the Y position. The circuit is then SW-1, M-1, M-5, SW-5, SW-6, R31b2 (relay pin 31b2), R31b1, M-2, M-4, ground. Note that the current is now going through the motor in the opposite direction, so the motor runs in reverse. Eventually the motor will reach the Park position and the microswitch changes to Z, thus breaking the ground path and stopping the motor.
So, failure to park can be caused by either the stalk switch or the relay. I believe we have more reports of switch failures than relay failures. So, I recommend you test the switch.
To test the switch, disconnect its harness and do some continuity tests. In the Off position SW-5 and SQ-6 should be shorted, and likewise for SW-1 and SW-7. If it fails these tests, replace the switch. Otherwise, replace the relay.
Note that I’ve never worked with your particular system. What I know on the matter is from spending the last several months retrofitting a ’89-’92 XJ-S Electrolux wiper system to my ’76. In the process, I’ve done a lot of circuit tracing and testing like this. BTW, I had three bad switches. Two were used eBay purchases, and one was brand new. Fortunately, the problem with the new one was simply that the single-wipe didn’t work. I am currently using it anyway since I can do without that feature.
'76 XJ-S coupe, red