1)
What is "Al Panai"?
1.
Radak: It is in Yerushalayim. They did abominations also in the Beis ha'Mikdash!
2)
Why do they sacrifice in gardens?
1.
Rashi: They set up the idolatry in their gardens, and there they burn fragrances "Al ha'Levenim" (on the bricks).
2.
Radak: So was the custom - "v'Sachas Kol Etz Ra'anan" (Devarim 12:2).
3.
Malbim: They hide themselves from people, and fear to do so in public. In front of people, they feign to fear Hashem.
3)
Why does it say "Al ha'Levenim"?
1.
Rashi: Refer to 65:3:2:1.
2.
Radak: When they burned [to make] bricks, they put on them incense to idolatry.
3.
Malbim: They offer incense on them so others will not know.