Revising this thread as I too am the (un)lucky owner of a hood tach purchased from the Parts Place, mfr'd in 2007.
I bought it mid-way through the year in 2013, installed it, and it worked beautifully throughout the summer of 2014.

Unfortunately, after washing the car one day in early 2015, I noticed the tach was behaving erratically; it was jumping between zero and the actual rpms. I quickly disconnected the tach, with the assumption that some water had perhaps found its way to the bottom side of the tach, i.e. between the hood and the tach's case. I never sprayed water directly at the face of the tach, but perhaps some humidity was affecting it?

I reconnected the tach after a couple days, with the assumption that it would have dried out, and it worked fine for a few weeks. From this time forward, I always disconnected it when washing for a few days. Nevertheless, after a few more weeks, it started behaving oddly, as the needle would move, but it wasn't responding to the actual RPM's. Disconnecting and reconnecting it would often correct the issue. Late in the summer, it stopped working completely; the tach lights up, and all connections are good right to the circuit board, but the needle sits at zero.

Enough of my tale of woe. My question to those of you who purchased units from Classic Industries: do you take any precautions when washing the car, or doing anything else? Thanks in advance.