There are 4 phillips head screws holding the heater box to the body.  Remove
these, and then you should have enough room to pull the box out and get to
those tabs that need to be cut on the back.  These screws are there (the
upper ones are hard to see).  Hope this helps.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242