coolant foaming

You don't mention the year of your Shogun.

If the engine is running at normal operating temperature, and the heaters are not getting hot, the obvious conclusion is that there is no (or very inadequate) coolant flow to the heater cores. So the question is why?

I'm not familiar with the Shogun, but all the Mitsu's I've worked on have the heaters supplied by 1" or so diameter hoses from the head or thermostat house area. You also mention the foaming coolant.

Check the following, and you may need to drain and refill the coolant to do them

look at the hoses from the engine to the firewall. If any on these feel soft or squishy, rather than hard (but not rigid), I'd suspect that the hose is squashing flat when coolant is circulating in the engine,

When the engine is at operating temp, with the heater control on full, are the heater hoses hot? If not, then there's a blockage somewhere.
Verify that the heater control valve is actually turning the valve. You'll need to look under the dash to do this. If the dash control is mechanical, there's a rod or wire that turns the valve. If electronic, there's a valve motor. Either way, you should be able to feel heat in the valve area. If the valve is not being opened, fix the problem.

If the coolant system has never been flushed, the heater hoses and/or cores may be blocked with rusty sludge. The best way to clean this out is to disconnect the heater hoses from the engine (ie drain the coolant) , and using a lawn hose in the RETURN line, backflush the heater system. Make sure that the dash control is on full.

Refill the engine with water, bleed and bring to operating temperature. Let it cool, drain again and then refill with the proper coolant mixture. This should get rid of most of the sludge build up.