When I did it, I pinched my wife's kitchen basting syringe and drained the coolant from the headers tank. I then disconnected everything placed the new header with float alarm and refilled with the coolant I removed. I then spent twenty minutes cleaning it of any evidence and finger prints.
I have flushed the fuel system this week! Drove 200 miles on Sunday, 300 miles on Monday and another 200 on Tuesday, TF is going well, going for a wheel refurb next week!
This thread can assist advise and entertain and all comments and posts are taken as that. Our own actions dictate who we are not what we read, that is unless specific guidance is requested. Although I am an Incorporated Engineer, in my time I have made many errors of engineering judgement but learning is the name of the game. Life is an education. Death even more so! Don't strop posting your experiences< I for one really enjoy the updates.
If there is a poor earth ie high resistance in the window circuit other paths to earth will be sought, it can throw up some odd results try checking the earth to your window switch'. I'm not a TF electric expert but many vehicles have a common earth site if this has corrosion it can throw up these types of issues.