1990 240DL - 163.5k miles

I posted awhile back concerning a screwy temp guage. I am finding that on colder and/or damp mornings the temp needle sits very low on the guage, almost no reading actually. After parking the car at work (in the sun typically) for a few hours and driving home for lunch (10 minutes) the guage pops up to just below mid-point.

I recently did the following:
** changed fan clutch (this fixed the A/C overheating, guage doesn't move from the ~mid-point...woohoo!)
** replaced all cooling/heater hoses
** replaced the temp guage (ECT?), the one that communicates with the engine mgmt system.
** Thermostat (92deg)
** Air box thermostat (the other one was hosed)

Didn't realize that there was another temp sensor for the guage, thank GOD it's easier to change the the ECT (is that what it's called? the other guage under #3). Anyhow, the guage temp sensor no longer has the plastic covering over the connector, fell to pieces when I touched it. I'm thinking this might have some strange effect with the cold/moist air and no guage reading. From a previous post, it was also recommended to check the sending unit behind the dash...haven't done that yet, but plan to.

My questions:
** Where should I start first? Guage sensor replace? or pull the dash?
** Is there any data from the guage sensor going to the ECU, how about data from the ECT to the dash guage?

Thanks for your advice.
Bean