If there was a "short" I would think there would be obvious burn marks showing you exactly where the problem was, at least with the 1st one, if it was power related. If it was a signal that was shorted, I would think just one signal, say the water temp, wouldn't be working. I really don't know much about these units, so just understand I'm just trying to help work thru this problem you have. When you "hit the dash" and it worked, it tells me you probably have a bad/intermittant connection. Since the 2nd unit failed in 30 seconds, I'm wondering if the unit that it plugs into, a "computer module" if you will, is possibly the culprit? Maybe it has a bad/intermittant connector or it's just flakey? Is there such a unit?
Just some thoughts, trying to help