[Oldsguy]

With the newer systems being controlled by their own microprocessors I would suspect that instead of the motor itself. You may also have a poor wiring connection which is causing intermittent operation. Those kind of problems are hard to troubleshoot without actually being there. You basically have your controls, the wiring, and the devices controlled (motor). Without replacement parts to substitute, and it would be a pain to do it that way, you are left with a good physical inspection. And I never liked getting up under a dash....but good luck.