Can you tell if they are "staying on" in intermittent or Low speed? When they "stay on" watch for the little pause at the bottom that indicates they are in minimum delay intermittent. Rotate the switch to Low, and observe if the little pause ceases. Rotate back to Off, and if they still run, and the little pause is back, the board is commanding "minimum delay Intermittent".
If the wipers are stuck on in Low (or High), this is not a board problem. Low and High speeds are directly from the switch to the motor. At the base of the steering column several wire bundles exit. On the LH side there is a clear connector with a "loop" of pink wire back into itself. This is the wiper switch. Disconnect it, and if the wipers stop, it is a switch problem. If they continue to run in Low (or High) it is the relay on the board or a mechanical problem with the "wiper park" switch which is internal to the motor.
If the "stuck on" is indeed in intermittent, along with the mystery wipe, you can probably cure both with an updated wiper switch which has different resistance in it. Cleaning the motor mounting bolt and ground strap (lower inboard bolt) may also help. The board responds to different resistance input from the switch to determine intermittent operation and delay, and washer pump operation. If there is any resistance in ground connections, this can confuse the board.
I once made HUGE gains on solving the mystery wipe in my DD by cleaning the corroded connector at the base of the steering column. It stayed away for a long time after the cleaning, but it eventually came back.
The updated switch is a cure-all, but it was about a $125 part back in the day, and I'm not sure it is still available. Replacing it is also very fussy as it requires some steering column disassembly to access. You'll need a steering wheel puller, a GM lock-plate puller, and a lot of patience.
TLDR version: Remove the WIPER fuse. Fab up a SPST toggle switch with an inline fuse holder and .250 male tab connectors on the wires. Insert them into the fuse box terminals. Leave the switch OFF unless you need the wipers.
Cheers
