Hi Doug,

A little surfing finds the Window Washer Reservoir:

https://www.tascaparts.com/v-2003-volvo-s40--base--1-9l-l4-gas/body--wiper-and-washer-components

See part 13: Level sensor 30859455

The bad news is it's NLA (no longer available from Volvo)

If you can't see and access the sensor you pull the tank:

The Good news is that you wont need to schlep under the car, the tank
slides down into clips to hold it. You pull it straight up being careful
not to damage the two wire harness to the level sensor or the rubber hose
attached to the pump. I don't know if you will need to move anything in the way to pull it up and out.

The level sensor is 1/3 the way up from the bottom and attaches with a screw on cover. Maybe the connector or wires are shorted?

More good news: FCP has the sensor $30.89 if you don't get lucky above.

While you have it apart check that the pump hose isn't cracked and starting to rot. The washer pumps crap out so you'll also pull the tank to replace a dead pump.

Report back with any good news.

It should be an easy fix. Bill