The AC compressor should come on for defrost mode - regardless of the temperature setting. This help to ensure dry air is delivered to the inside of the windshield. If the air were humid (damp) it would condense when hitting the cold glass - cause frost to form almost immediately on the inside of the windshield.

There should be a couple of hoses that run into the firewall of your truck to connect to the inlet & outlet of the heater core itself. When the engine is at normal operating temperature (depending on outdoor temperature this could be 5 - 15 min or more at idle - less if being driven), these hoses should be as hot as the radiator hose. If they're not, it would explain why you don't get any heat in the cab. Repair or replace any hose that is pinched, kinked or crushed.

Make sure that the cooling system is FULL. This is done by checking the coolant level in BOTH the overflow reservoir AND radiator itself. There is a very real scalding danger if you attempt to remove the cap of a radiator when hot. Allow several hours of cooling before attempting. With the engine off AND cold, open the radiator cap and inspect the level. It should be right up to the bottom rim of the filler neck. Set the heat temperature to HOT and the fan to HIGH. Start the engine and let it idle. While it is warming up, check the coolant overflow reservoir level. It should up to the COLD level mark. Top it off as needed with a 50/50 mixture of water and compatible coolant for your truck, and recap. Check the coolant level in the radiator again. If it appears full, gently increase the engine RPM be stepping on and off the gas pedal a few times to help dislodge any air bubble in the system that may be present (or have a helper do it while you watch the coolant level in the radiator fall and rise). This is only effective if the engine is at normal operating temperature (thermostat is OPEN). Return to the radiator an check the level again. Slowly add a 50/50 mixture of water and compatible coolant for your truck as needed. Rev the engine slightly a couple of times again to help ensure any air in the system has been displaced and top off with coolant as needed - close the radiator with the cap. Note the overflow reservoir level and allow the engine to reach normal running temperature. Once the engine has begun to warm up, DO NOT remove the radiator cap again until the engine has been allowed to cool completely. You should be able to be able to see the coolant level in the reservoir if the coolant in the radiator has been brought up correctly. The cooland level in the overflow reservoir should now be at HOT. If it is at any level lower than the HOT mark, top it off with a 50/50 mixture of water and compatible coolant for your truck. If the reservoir was emptied completely while running the engine, you should stop, allow the engine to cool and start the process over again. Once you are sure the coolant level in the radiator is full, and the overflow reservoir is never allowed to go empty, you can monitor the level and add coolant to the system via the overflow reservoir. This means the the entire system is flooded with coolant - and no air "pockets" exist.

If still unable to get heat, it may be a result of a clogged hose or heater core, or a stubborn air pocket in the heater core itself or inlet / outlet hose. With an empty bucket or container & cold engine, you could remove one hose and pinch or close both openings. Next, open slowly / briefly see if antifreeze flows FROM the heater core to the end of the hose while directing the flow into the empty bucket / container. Allow the coolant to drain until you're satisfied that any trapped air has been pushed out by coolant from the radiator thru the hoses and heater core. Reconnect hose.

Good luck!