[Mike_The_Grad]

MAV,
Are you testing the wipers with the engine running? Do you have a voltmeter wired into your truck so that it displays system voltage anytime the key is turned, or are you using the factory ammeter?
I ask because I have a voltmeter wired into my system and no longer utilize the factory ammeter (the new American Autowire "Classic Update" kit I installed doesn't have provisions for the factory ammeter.) When I insert the key and turn it to the "run" position I usually see a reading of 12.5V on the voltmeter, when I start the truck and is idling I usually see just to the right (highside) of 14V. When I turn the wiper switch to the first detent (Low), it'll drop to around 13.75V.
Now, wuth engine idling, and my headlights on High, wipers on High, Heater blower motor on High, 4-way flashers on, and stereo at MAX, my voltmeter inside the truck reads 13.2V.
I just took this reading about 3 days ago because I thought I was having power issues somewhere causing my stereo to intermittently loose power, but yet immediately regain the power as if nothing happend. ( turned out to be RF/EMI emissions from something under the hood. I placed a Ferrite Clamp over the radio power and constant batt. Power wires and no more intermittent power loss.)
Anyways what I'm getting at is, you are going to have voltage drop In any electrical system. Anything less than 1 Volt of drop is considered "Normal",for the most part.