Is there a place for fresh air entry into the cab of my truck

cab heat dead.
NO CAR TOLD AT ALL (MAKER, MODEL, YEAR)
you skipped test #1 find out what end of car is bay
engine side or cab side, cutting the job in half saving all that extra cash thrown.
testing 1,2,3.
hot engine, say drive 10miles/
I use my hand as a tool or my IR thermal gun.
if hand flies off too hot is GOOD temp, IR GUN is hands free.
To the thermostat housing and Rad hot end goes to 180f to 200F
yes then all that is good.
then the 2 tubes going through the fire wall. is next.
heat core cab tubes/hoses.
no water valve there, no need test it. Look for bypass hose here if you bought a car with no heat,(called looped, due to core leakage hack.)

the 2 tubes are at 180f is good one cold is bad.so core needs a backflush. if one is cool or luke warm.
ok both tubes hot the CAB side is bad, I just saved you big cash.
next we test the dampers, called blend doors.
if stuck closed the heater core air path is blocked so heat is gone.
make sure recirc door is active (no fresh air ) makes max heat.
top cause bad damper actuator. motor,, no car told so not possible tell more. now. 100 years of heater, I can not guess.

Replacement of the heater core is not usually necessary unless it is leaking -- you can see or smell coolant inside the cab.
When the engine is running and warmed up, with the heat set for full hot, are both heater hoses hot? If not, you can try back flushing the heater core. There are several good videos on YouTube showing how to do this.
If both hose are hot but no heat, with the controls NOT in the recirculate mode, is there (cold) air movement through the vents inside the cab? If no air movement, your air intake plenum (area under the grill below the wiper blades) may be plugged up, blocking air intake to the blower fan.

Just for fun,clamp off the heater hoses,just squeeze them flat in place to stop the hot coolant flow through the heater core.Take it for a spin and see if the a/c stays cold.Sometimes with increased hot coolant flow through the heater core with increased engine rpm (driving) and a leaky air flow door in the a/c heater assembly,it can be hard to keep the a/c air cold.

The air from the heater core will be as hot as the coolant temperature of the engine. 80 Degrees is the opening temperature of the thermostat and that will be where it remains until the engine starts to work or the ambient temperature rises.
Thermostats come in a couple of different opening temperatures so check around for one that opens around 85 degrees if you want hotter air in the cab
Another point is to close the recirculating vent door to the cabin as the heater core is trying to heat fresh freezing air if it is open

Get a higher temperature thermostat. Fluid in the block warms up first and hotter then mixes with cold radiator fluid this lets it warm faster. Your current thermostat is opening too soon preventing it from warming the whole coolant system,also put a piece of cardboard across the grill to keep the radiator warmer.

First thing I would do is flush the heater core with a 4 to 1 mixture of water and muratic acid. Put it in there and blow it out both ways 3 or 4 times. Let the acid mix set for about 10 minutes before blowing it out with air. This is a cheap fix, if it works. $7 and your time. Got to say though, it only works about 30% of the time. The other 70% is to remove the heater core. Dodges/Chrysler su(k. To get to the heater core, you have to drop the steering column, and remove the entire dash. Then remove the heater/A/C plenum to remove the heater core. Buy a manual to do this. Way to many hidden screws and bolts. This job costs about $800 at a garage. It takes about 8 hours to do, if you know what your doing.

What kind of setting do you have? is it turn knob, push button or something else?

Sounds like it's the air control flap which determines the mix of heated and fresh air. This could be vacuum controlled or manually with a cable.

There are several potential issues to consider. First, check your antifreeze level--making sure you don't open the radiator cap when hot, of course. Your cab heater uses engine antifreeze for a heat source, and when the level is low, there may not be enough fluid to reach the heater core. This is the most common reason for poor heat output in the cab.

If that checks out, it is also possible that you have clogged or collapsed hoses leading to the heater core, or that the heater core (located in the cab) is clogged. This can occur when antifreeze has not been changed often enough, when internal engine rust had built up, or especially when a "radiator seal" product has been added to the antifreeze.

It is also possible that nothing is wrong with the heater core but that the various controls, ductwork, or dampers for your truck have become jammed or broken and that the system is allowing a mix of cold air with the hot air--in spite of your setting asking for all hot air. Since many modern cars now have electronic controls for the heat, this can be difficult to judge without disassembly.

if the inlet and outlet hoses are getting hot to the heater core then the problem is the air temperture control blend door actuator in the heating/A/C ducts, this door directs air flow either across the heater core or the AC evaporator or mixes the 2 air flows for a desired temp at the outlets, hence the name blend door, to change this part u must remove the dash assembly.