5 years ago, the same problem happened to my 88 240DL. Initially, I could not figure out why. After replacing the defroster switch--genuine Volvo 1324679 (about $13), I solved the problem. Good luck!

I'm thinking the relay is accessible from the driver's footwell, but this is not confirmation. You'll recognize it by the wire colors at its socket: Yel, blk, yel/red, blu/blk.

But it may be quicker continuing your pursuit with the voltmeter, instead of guessing about parts. Verify the voltage at the grid terminals.

On the other hand, if your 1989 240 is like one of our two 1989 240's, it will be a 245, and the problem (by likelihood) is the wiring in the tailgate hinge. But if yours is a 244, like the other of our two 1989 240's, and you see no juice getting to the grid, the relay is a suspect by likelihood: look for cracked solder on the circuit board inside it, or cripsy socket from overheating oxidized fastons.
--
Art Benstein near Baltimore

A clear conscience is usually the sign of a bad memory.

Look for clogs at at any junctions or sharp curves in the hose.