Home Link

Need to know if part is available, & where for my HomeLink on my Gen. 3 IV Prius. Just the other day I pushed one of the three buttons as I backed out of my garage & the "button" fell out! Found it that first time under my drivers seat, but later it fell out again and has disappeared! What has happened is the button has broken. I'm wondering if a replacement "button" can be obtained for the unit. The HomeLink works fine, just missing one of the three buttons. The other two are in use for another purpose, so it's not an option to use one of the other ones. I'm sure another unbroken button would just "pop" into place, but don't know where to find one. Can anyone help??