Well, as to the 58d, I'm pretty sure that is the illumination input. It should go up when the lights are on and the dash illumination is set highest.
The early and late systems are definitely different. The biggest change being the size of the display. Being at home I can't lay out any lists but the extent of diagnostics goes beyond actual values and fault codes to the activation of air flaps in the dash.
I wrote an article for "Import Car". It can be viewed at my site:
http://www.continentalimports.com/ser_ic40142.html
The article is specific to a 97 210 chassis, but the information is similar for all chassis.