The switch works, turning on the light. The leads do look connected to the rear windshield, and there is power running through the leads; I tested it with a multimeter. However, it doesn’t seem to defrost, and using a multimeter on the conductive paint itself doesn’t seem to yield any results. 

What could be wrong here?

Its toast!  Just not as warm...  Power arrives at the panel, and I assume you tested an earth leading away, and you can't get a reading through the panel itself.

If the earth wire goes to the chassis with less than 5ohms, the problem is in the panel, and most likely there is a break or corrosion in the wire somewhere along the way. I don't know what resistance it should show, but 10ohms would be enough to defrost a window.

You need to test power going into the element, power coming out of the element.

Also the quality of the earth.

If they are good, it has to be the element.  Back in the day you could but element repair kits, which was basically something you painted on over the break - once you found it.

Here you go, can still buy it: https://www.ebay.com.au/itm/126223599329?chn=ps&_ul=AU&_trkparms=ispr%3D1&amdata=enc%3A1kD6G9jjDT8mvYIenkS9Kpw57&norover=1&mkevt=1&mkrid=705-139619-5960-0&mkcid=2&mkscid=101&itemid=126223599329&targetid=1740279243329&device=c&mktype=pla&googleloc=9071325&poi=&campaignid=18277121424&mkgroupid=142426603833&rlsatarget=pla-1740279243329&abcId=9301724&merchantid=5320211051&gclid=Cj0KCQiAqsitBhDlARIsAGMR1RheL8-GxprntG8HPrWh8ksujXbWJuh-2fcv6kxWGUVvj3UVP23GEVkaAlMkEALw_wcB