I am installing a windshield washer fluid pump. I have a rear window defroster that doesn't work, and that I have no plans to use/fix. I want to use that switch to power the pump. Do I need to use a relay, or can the switch handle that kind of load for short bursts?

you can modify or replace the actual wiper stock on the column to incorporate a switch onto the control stock

Not on an early car, you can't...

The defroster switch is made to run a relay feeding a pretty good-sized current draw. Even though the motor might not be as large a load (and definitely not as continuous as a defroster) I would still play it safe by using a relay.

--DD