Back story... the hs worked great up until the last couple of frosts earlier this year but then stopped. Decided to investigate this the other day. Both relays clicked so off with the plenum (much quicker to say then do!) And a poke with a dmm showed healthy voltage at both white connectors (disconnected) using both the battery and the ground stud for the hs. The ground stud looked a tad rusty though so I went to remove the nut and the stud sheared off. I'm hoping (bear with me here, I'm keen but green with leccy stuff!) that the stud was maybe causing high resistance and therefore the becm wasn't seeing the necessary load so it wouldn't work the hs. The ground stud is the only common connection under the plenum for both sides of the hs and as they both stopped working at the same time that sounds plausible to me.
That ground stud is now all repaired and finally to my question.
How can I test that the screen is now working? I have nano and a dmm and enthusiasm but that's it.
Googling about has led me to believe that a clamp meter may be useful?