Look under the hood in the PDC for a 5 amp fuse, I think it is fuse number 2 if I remember right, this is the "M1" feed which is a non switched circuit for memory, interior lights, Power mirrors, etc.
Without fail the most common cause of blowing this fuse when doing a heater core is hitting the glove box light wiring on the dash frame if you left the battery hooked up when undoing that side..If you had the battery unhooked, and that fuse is still blown, That circuit is a Pink wire....start looking for pinched or rubbed thru stuff on that side.

Rick