Cooling System Question

I have a 2001 Winstar Sport with 125,000 KM. I have limited front heat. I have been checked out the actuator & the blend door, coolant, flushed the system twice, had the stuck open thermostat changed. I was thinking the only thing left is a partially plugged heater core b/c the first time I had the coolant changed it was really dirty with lots of crap in it. I do get rear heat by the way and the mechanic suggested this is b/c we probably were not using the rear heater much which is true. Any comments on the possibility of a weak water pump just not circulating much fluid?

Can the flow or pressure of the system be tested to confirm the pump is doing what it is supposed to? If so, how and what should it be?

I have no coolant loss issues, just not enough front heat to keep up when it get to -20C...I freeze my ass off! I was planning on pullling out the heater core soon, but thought maybe someone has other ideas I could confirm before that job? By the way, does anyone have a procedure or pictures for the removal of the heater core? I found some basic instructions for a 98 which basically say to pull the hoses off the fire wall (remove the wiper first for better access) and then remove the centre console and then the heater core box is right there. BUT it doesn't say how to open the heater box? Seems simple enough, but I am sure it isn't!

A coolant pressure tester wont check flow. You can get a LAzer pointer temp gauge to check your heater hoses to the dash core right at the firewall. You can also, just pull the two hoses off at the firewall and try to run water thru the core with a garden hose etc. If its not flowing well, then you know its gotta get pulled out and replaced.

If the heater is off inside BLOWER, the temp on both sides of the hoses should be the same. If its clogged it will be cold on one end, or way cooler. Remember Heater blower off, if its on, it will just cool the coolant when it flows back to the engine.

If you just flushed your system or replaced your thermostat. Try bleading the system a few time with the Bottle cap off. Fill it, run engine hot, and keep squezin your Upper rad hose until your thermostat is wide open HOT. Then top off..