I just unplugged mine at the firewall and adapted a garden hose to flush it in reverse, then both ways. I did a cylinder head job on it a while back and I think there was some trash that got into the cooling passage and it lodged in the heater. Heat works great now. I had bypassed my rear heater since it never gets that cold in North Carolina.Any DYI’s out there or just as simple as unplugging from the firewall and sticking a hose in it? Maybe a chemical?