Those pre-wired LEDs are nice because the bezels give your project a handsome finished-look. I'd be surprised if you can find such pre-wired LEDs for 18V AC operation. Typically these are 12V DC common for auto DIY projects. I see they are about $1 each on Amazon but you have to buy a handful. In the photo below you can see how these 12V DC LEDs have an embedded resistor. An inquiring mind might ask whether this means you just add a diode (to convert AC to DC) and off you go?
Modifying a pre-wired LED sort of defeats the intent. If messing at the component level anyway with the soldering iron ready to go, you can buy LED bezels and make exactly what you want. Again you run into the minimum order quantity conundrum if you only need one or two.
