That's correct so you have to modify the mirrors you have. If you're not up to it you might try calling a glass shop or local mechanic about installation.
These are the parts
Heating pads: Burco Redi Heat #3801 (size 3 ¼” x 5 ¾” > recommend 5” x 7”)
Burco | Home
Wire Harness: Muth #H-MOD (harness, switch, This kit includes the built-in 10 minute defroster timer.)
Muth Mirror Systems, LLC - Automotive Signal Mirror® Kits