1996 Dodge Ram heater

Couple thoughts:

The thermostat may be sticking open and not allowing it to heat up like it's supposed to.

There is an air pocket that may be in the heater core.

The water pump may be a problem.

Questions:
What liter size is the engine?

Is the water pump new or reman?

Was the heat present before the work?

Did you overheat it at all, gauge going into the red?

Feel the heater hoses at the core are they hot going in and hot coming out or just luke warm coming out?
Should be hot on both sides.

IF you have access to a heat temp gun shoot the rad from top to bottom as there have been a common thread of radiators plugging at the bottom.

Did you copmpare the new pump to the old one to make sure the fins were not a reverse directional?

Take it back to the shop, that is thier baby to figure out.

They may need to bleed the system. Hopefully they didn't put a reman one on as the fins can be corroded and not changed by the reman company or a reverse fin put on.