would be best to check the actual voltage at the battery
when the voltmeter jumps up to make sure the problem
is not just a faulty gauge,could also observe the lights
to see if they brighten when the volts jump up-if so,
most likely an alternator problem.The reason the add-on
manual fan switch is burning out is because a relay is
supposed to be used to switch the power to the fan
motors(there is probably at least 10 other posts with the
exact same direct-wired manual switch burnout problem

)It's pretty easy to rig up an add-on thermostatic
fan control that will be reliable