I removed the wiper motor from the linkage this morning, and ran it in the engine bay. The motor's shaft turns good at both speeds. So I crawled under the dashboard to have a look at the wiper linkage.

The linkage has a long connecting rod between the two wipers, but it refuses to budge (even though it should move from side to side as the motor turns a crank). I looked at both ends and all the linkage appears to be intact. Could it be that it got shoved beyond its normal range at some point?

All the pivot points appear to be in good condition, although two of them on the passenger side appear loose. Are there any loose-looking pivot points on 'good' wiper linkage (they are visible under the dash from the passenger footwell)?