Just changed the rear tire and need help getting the rear break back on 1999 heritage softtail

If you have the wheel properly installed, putting the rear brake back on the bike should not be a problem. The brake caliper bracket should act as the wheel spacer on the right side of the bike and must be properly positioned in order to get the axle back in.

With the caliper bracket on the bike, make sure the brake pads are properly positioned in the caliper bracket. Double check to see that the pads are not installed backwards, especially on the back side of the rotor. Carefully, slide the caliper down over the pads, without distrurbing them, and into the caliper bracket. If this is giving you a problem, you may have to press the piston in the caliper back into the the caliper just a bit to clear the pads. If you replaced the pads with new pads, press the piston all the way back into the caliper. I usually use a large pair of slip joint pliers or a large C-clamp to do this with. Make sure you put a rag or something around the caliper to protect the paint. Good Luck!