Well, you said the heater didn't work. This means there is no warm air
coming from the airbox.

What you meant to say is the BLOWER isn't working. Big difference. If the
heater weren't working it means there's something wrong with the car's
cooling system: a bubble in the system, a clogged hose, too little
anti-freeze, etc.

The blower not working is quite a different scenario. This is more of an
electrical problem with the control in the dash or the blower behind the
glove box.

Since you said slamming the door made it work, I'd be looking for a loose
connection somewhere. Odd it was the DS door, since the blower is located
on the PS of the vehicle.

You mentioned the "heater" but it sounds like you have a problem with the
blower.  I asked those questions because the information was not quite clear
to me, and the information I requested will help me or a competent
technician diagnose the cause of the problem, or at least point you to some
things to check.

The "heater" works by allowing hot engine coolant to flow through the heater
core, and the temperature setting on the dashboard regulates how much
coolant flows through the heater core.  A problem with the heater would
relate to coolant flow through the heater control valve and heater core.

The blower is the fan under the passenger and moves air past the heater
core.

The damper system regulates where the air moving past the heater core comes
from and at where the air comes into the passenger compartment.  Some people
describe the air outlets as "high," meaning the dashboard vents, "medium,"
meaning a mix from the dashboard and floor vents, and "low," meaning the
floor vents.

Rather than have you check stuff that does not relate to the cause of the
problem, I asked for a clarification.  I've listened to literally thousands
of people describe problems with their cars and have learned that people
have different ways of describing those problems.  Most of those people
don't like checking or paying someone to check something that had nothing to
do with what they were trying to describe, so asking additional questions
for clarification helps avoid this.

I generally try not to ask irrelevant questions, although I'll admit that
some may seem so sometimes.

Mileage is almost always a relevant question when talking about a problem
with a vehicle because it provides clues as to how much use the vehicle and
the components in the vehicle have had.  In this case, mileage could mean
something on this issue if it is related to the heater, blower motor, and
blower resistor pack.  The coolant questions are not as relevant if you are
talking about the blower or dampers, but if you are talking about the
heater, then they can mean something on this issue.

Also, are there any aftermarket electrical accessories installed, like an
audio system, remote starter, security system, etc.?  Whoever installed the
accessory may have double-tapped one or more terminals in the junction block
and loosened a relay, or a wire splice may have worked itself loose, and
slamming the door may have jiggled the loose connection.