After spending most of a day checking all functions of all front window switches (with a schematic and ohm meter), all voltages and grounds, resistance of the window motors (2.5 ohms measured at the window switch connectors), I came to the conclusion nothing was electrically wrong with the power window circuits. That same evening I stumbled on this web site and read where someone on another year-model Caravan had been able to fix my exact symptom by holding the window switch in the "lower" position while slamming the door several times. The next day, with the ignition switch on "Acc", I tried this with no success. Then I tried it again with the engine running (raising the voltage to the window motors by about 3 volts) and each window started working on about the second or third slam.

I have no idea how long this "fix" will last, hopefully long enough to get rid it. In any case, I will try this as many times as necessary before replacing the regulator assemblies.

Sometimes this van sits unused for extended periods (6-8 weeks) and I suspect this has contributed to the problem.