Follow up: once I had this part, I noticed that one side of the brush head (where the white plastic tabs are clipped into the metal case) had popped out on its own. I put the head back in place put everything back together and now it functions properly.

this is all great instruction on how to get this apart and to get that head out BUT there is no link to buy the replacement head in the USA. A gentleman posted a link, but it’s for eBay only sold in the UK.

Bummer

I wrapped the open end of the base with cardboard and then with channel locks squeezed on the two short sides while pushing down on the metal brush head on a hard surface and it popped right out