Where do I get this part (just the plastic cap)

Cheapest/easiest will be from either a dismantler or one of the sticky companies if they have one laying around.

 

Ricambi is great. I used them for major kits and quick turn around on needed parts.

The PN you need is 215406 but I dont think you can just buy the top alone, you need to buy the whole 12V assembly.

You had asked for just the cap alone...

 

https://www.ricambiamerica.com/215406-12v-socket-not-for.html

 

The entire part is $40. In Ferrari land thats a rounding error. Hardly worth the time looking any further if you ask me.