My wipers ('85 300SD) have begun to decide that, after some period of normal operation, they deserve a nap. Selecting LOW speed leaves them still in the "parked" position. Hmm... Momentary selecting HIGH speed produces high-speed wiping. LOW speed wiping and intermittent wiping then resume normal operation. Then, when left on intermittent, the wipers go back to their nap...
(Side note: at highway speeds I don't really use the wipers, as there is a TERRIFIC airflow pattern which just pushes the rain away, leaving relatively clear vision without the annoying sweep! Only when in the draft of another car, which interrupts the airflow, do I need wipers...)
Anyway, back to the main subject.
For further information, the wipers have, once or twice, stopped in their tracks on the glass when I switched to OFF . AHA!
MY deduction, educated guess after studying the wiper schematic is there is a problem in the wiper PARK contact path, which is (somehow) overcome by HIGH speed voltage.
Assuming that a good cleaning of this contact is the solution, is the wiper motor easy to remove, disassemble, clean & reinstall, or is it better to remove and replace?
I had a similar experience with the Suburban, but the wipers would go completely non-functional until a big bump, or rapping on wiper motor, re-started them (for a while.) It turned out that the printed circuit control board, in the motor housing, was defective. I replaced the board, which then cured this problem.
Sorry to be long-winded
'96 E300D 56k mi
'95 Audi 90 118k mi
'92 GMC Suburban 138k mi
'85 300SD 228k mi