Help BRRRRpart 2 Climate Control won't

[Mark and Aviva Woodland]

Patient is an '86 5KTQA w/ 230k on the clock.
The Climate Control head went dark, and the blower won't..
Got a chance for a few minutes diagnostics, and all fuses check out OK, the
C.C. head plug shows a good ground, 14+ volts while charging, and 12.5 volt
signal, and swapping out the Climate Control head with another one yielded
the same results.. nothing. No display, no Defrost, no nothin'.
Has anyone encountered this before? When I get a few more minutes, I'll dive
into the big books for the formal diagnostic routine, but in the meantime,
any BTDTs would be appreciated. Most diagnostics are predicated on a
'diagnostic display code', and that's not one of the options.
Merry Christmas to them that do, and Happy Holidays to all the rest
Mark