Very odd indeed. Is there any aftermarket interior lighting installed? EL gauged in particular....?
Confusing speculative, not completely thought out banter to follow:
The two circuits aren't connected at all. There is no illum/dim integration for this display (one of the only lighted circuits like this in the MKIII, the only other being the light switch indicator).
I can't find the blue TEMS display I've got somewhere around here for reference, but I believe the black wire chould be ground full time and the red, blue and green wires positive only depending on the TEMS controller's response. Which leaves us two possibilities to explore: The TEMS is active full time when the three are lit up (this is unlikely as ALL three conductors would have to be shorted across a positive signal) OR there's a bridged wire in the cluster or harness. Are you sure the TEMS isn't going full firm when the three lights light up? If so the bridge would be at one of the signal input lines (SPD, SS1, SS2 or VS).
http://www.cygnusx1.net/Supra/Librar...x?S=Main&P=115
Sorry if this is confusing.... in short, there's no easy way for this to be occuring, but the more likely of the unlikely is that one of the input signal lines is shorted to a positive.... wierd.