Ah yes i used to work on daewoo's and chevrolets the pipes to get off the heater matrix are abit of a pain!its a dash out job!not to bad but not to good!cant actually remember where the stat is tbh?what year is it?

If i was you take off the pipes to the matrix and give it a gentle blast with a hose pipe obversely you want to put the pipe on the outlet side have had a few that just get crusted up!