Well, it only took about an hour for the silly fan to stop working today. (Usually, it's a lot quicker than that).
Once the fan stopped blowing, I pulled the connector to the blower assembly and measured 13.5 V (the van was running), so I presume that means the blower assembly is the problem? I'm a little confused as to how the blower can be intermittent like this. It actually starts to slow down before it finally gives up altogether. That's not normal behavior for an electric motor. Is there more to the assembly than just a motor and fan?